diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -2445,6 +2445,7 @@
     'PhabricatorStandardCustomFieldBool' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldBool.php',
     'PhabricatorStandardCustomFieldCredential' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldCredential.php',
     'PhabricatorStandardCustomFieldDate' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php',
+    'PhabricatorStandardCustomFieldDecimal' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldDecimal.php',
     'PhabricatorStandardCustomFieldHeader' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldHeader.php',
     'PhabricatorStandardCustomFieldInt' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldInt.php',
     'PhabricatorStandardCustomFieldInterface' => 'infrastructure/customfield/interface/PhabricatorStandardCustomFieldInterface.php',
@@ -5735,6 +5736,7 @@
     'PhabricatorStandardCustomFieldBool' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldCredential' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldDate' => 'PhabricatorStandardCustomField',
+    'PhabricatorStandardCustomFieldDecimal' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldHeader' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldInt' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldPHIDs' => 'PhabricatorStandardCustomField',
diff --git a/src/applications/drydock/blueprint/DrydockAmazonEC2HostBlueprintImplementation.php b/src/applications/drydock/blueprint/DrydockAmazonEC2HostBlueprintImplementation.php
--- a/src/applications/drydock/blueprint/DrydockAmazonEC2HostBlueprintImplementation.php
+++ b/src/applications/drydock/blueprint/DrydockAmazonEC2HostBlueprintImplementation.php
@@ -90,8 +90,6 @@
 
     $settings = array(
       'ImageId' => $this->getDetail('ami'),
-      'MinCount' => 1,
-      'MaxCount' => 1,
       'KeyName' => $this->getAWSKeyPairName(),
       'InstanceType' => $this->getDetail('size'),
       'SubnetId' => $this->getDetail('subnet-id'),
@@ -111,14 +109,78 @@
       }
     }
 
-    $result = $this->getAWSEC2Future()
-      ->setRawAWSQuery(
-        'RunInstances',
-        $settings)
-      ->resolve();
+    if ($this->getDetail('spot-enabled') &&
+      $this->getDetail('spot-price') !== null) {
+
+      $spot_settings = array(
+        'SpotPrice' => $this->getDetail('spot-price'),
+        'InstanceCount' => 1,
+        'Type' => 'one-time',);
+
+      foreach ($settings as $key => $value) {
+        $spot_settings['LaunchSpecification.'.$key] = $value;
+      }
+
+      $result = $this->getAWSEC2Future()
+        ->setRawAWSQuery(
+          'RequestSpotInstances',
+          $spot_settings)
+        ->resolve();
 
-    $instance = $result->instancesSet->item[0];
-    $instance_id = (string)$instance->instanceId;
+      $spot_request = $result->spotInstanceRequestSet->item[0];
+      $spot_request_id = (string)$spot_request->spotInstanceRequestId;
+
+      // Wait until the spot instance request is fulfilled.
+      while (true) {
+        try {
+          $result = $this->getAWSEC2Future()
+            ->setRawAWSQuery(
+              'DescribeSpotInstanceRequests',
+              array(
+                'SpotInstanceRequestId.0' => $spot_request_id,))
+            ->resolve();
+        } catch (PhutilAWSException $ex) {
+          // AWS does not provide immediate consistency, so this may throw
+          // "spot request does not exist" right after requesting spot
+          // instances.
+          continue;
+        }
+
+        $spot_request = $result->spotInstanceRequestSet->item[0];
+
+        $spot_state = (string)$spot_request->state;
+
+        if ($spot_state == 'open') {
+          // We are waiting for the request to be fulfilled.
+          sleep(5);
+          continue;
+        } else if ($spot_state == 'active') {
+          // The request has been fulfilled and we now have an instance ID.
+          $instance_id = (string)$spot_request->instanceId;
+          break;
+        } else {
+          // The spot request is closed, cancelled or failed.
+          throw new Exception(
+            'Requested a spot instance, but the request is in state '.
+            '"'.$spot_state.'".  This may occur when the current bid '.
+            'price exceeds your maximum bid price ('.
+            $this->getDetail('spot-price').
+            ').');
+        }
+      }
+    } else {
+      $settings['MinCount'] = 1;
+      $settings['MaxCount'] = 1;
+
+      $result = $this->getAWSEC2Future()
+        ->setRawAWSQuery(
+          'RunInstances',
+          $settings)
+        ->resolve();
+
+      $instance = $result->instancesSet->item[0];
+      $instance_id = (string)$instance->instanceId;
+    }
 
     // Allocate the resource and place it into Pending status while
     // we wait for the instance to start.
@@ -531,6 +593,28 @@
           'By default, Phabricator will automatically install and configure '.
           'SSH on the Windows image.'),
       ),
+      'spot' => array(
+        'name' => pht('Spot Instances'),
+        'type' => 'header',
+      ),
+      'spot-enabled' => array(
+        'name' => pht('Use Spot Instances'),
+        'type' => 'bool',
+        'caption' => pht(
+          'Use spot instances when allocating EC2 instances.  Spot instances '.
+          'are cheaper, but can be terminated at any time (for example, in '.
+          'the middle of a Harbormaster build)'),
+      ),
+      'spot-price' => array(
+        'name' => pht('Maximum Bid'),
+        'type' => 'decimal',
+        'caption' => pht(
+          'The maximum bid to pay per hour when running spot instances.  If '.
+          'the current bid price exceeds this amount, then the instance will '.
+          'be terminated.  WARNING: You should not set this higher '.
+          'than the On Demand price for this instance type, or you could end '.
+          'up paying more than the non-spot instance price.'),
+      ),
     ) + parent::getFieldSpecifications();
   }
 
diff --git a/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDecimal.php b/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDecimal.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDecimal.php
@@ -0,0 +1,71 @@
+<?php
+
+final class PhabricatorStandardCustomFieldDecimal
+  extends PhabricatorStandardCustomField {
+
+  public function getFieldType() {
+    return 'decimal';
+  }
+
+  public function getValueForStorage() {
+    $value = $this->getFieldValue();
+    if (strlen($value)) {
+      return $value;
+    } else {
+      return null;
+    }
+  }
+
+  public function setValueFromStorage($value) {
+    if (strlen($value)) {
+      $value = (double)$value;
+    } else {
+      $value = null;
+    }
+    return $this->setFieldValue($value);
+  }
+
+
+  public function validateApplicationTransactions(
+    PhabricatorApplicationTransactionEditor $editor,
+    $type,
+    array $xactions) {
+
+    $errors = parent::validateApplicationTransactions(
+      $editor,
+      $type,
+      $xactions);
+
+    foreach ($xactions as $xaction) {
+      $value = $xaction->getNewValue();
+      if (strlen($value)) {
+        if (!preg_match('/^\d*\.?\d*$/', $value)) {
+          $errors[] = new PhabricatorApplicationTransactionValidationError(
+            $type,
+            pht('Invalid'),
+            pht('%s must be an decimal.', $this->getFieldName()),
+            $xaction);
+          $this->setFieldError(pht('Invalid'));
+        }
+      }
+    }
+
+    return $errors;
+  }
+
+  public function getApplicationTransactionHasEffect(
+    PhabricatorApplicationTransaction $xaction) {
+
+    $old = $xaction->getOldValue();
+    $new = $xaction->getNewValue();
+    if (!strlen($old) && strlen($new)) {
+      return true;
+    } else if (strlen($old) && !strlen($new)) {
+      return true;
+    } else {
+      return ((double)$old !== (double)$new);
+    }
+  }
+
+
+}