update cost info for cloud nat

Signed-off-by: Colin Simmons <colin.simmons@engineerbetter.com>

Author: crsimmons

docs/cost.md

@@ -10,19 +10,23 @@ By default, `control-tower` deploys to the AWS eu-west-1 (Ireland) region or the
 | Web Server    | t2.small         |     1 |       18.30 |
 | Worker        | m4.xlarge (spot) |     1 |      ~50.00 |
 | RDS instance  | db.t2.small      |     1 |       28.47 |
-| NAT Gateway   |         -        |     1 |       35.15 |
+| NAT Gateway*  |         -        |     1 |       35.15 |
 | gp2 storage   | 20GB (bosh, web) |     2 |        4.40 |
 | gp2 storage   | 200GB (worker)   |     1 |       22.00 |
 | **Total**     |                  |       |  **176.62** |
 
+> \* NAT gateway also incurs $0.048 per GB processed by the gateway (both ingress and egress)
+
 ## GCP
 
-| Component     | Size                              | Count | Price (USD) |
-|---------------|-----------------------------------|-------|------------:|
-| BOSH director | n1-standard-1                     |     1 |       26.73 |
-| Web Server    | n1-standard-1                     |     1 |       26.73 |
-| Worker        | n1-standard-4 (preemptible)       |     1 |       32.12 |
-| DB instance   | db-g1-small                       |     1 |       27.25 |
-| NAT Gateway   | n1-standard-1                     |     1 |       26.73 |
-| disk storage  | 20GB (bosh, web) + 200GB (worker) |   -   |       40.80 |
-| **Total**     |                                   |       |  **180.35** |
+| Component     | Size                                              | Count | Price (USD) |
+|---------------|---------------------------------------------------|-------|------------:|
+| BOSH director | n1-standard-1                                     |     1 |       26.73 |
+| Web Server    | n1-standard-1                                     |     1 |       26.73 |
+| Worker        | n1-standard-4 (preemptible)                       |     1 |       32.12 |
+| DB instance   | db-g1-small                                       |     1 |       27.25 |
+| NAT Gateway*  | [Cloud NAT](https://cloud.google.com/nat/pricing) |     1 |       35.04 |
+| disk storage  | 20GB (bosh, web) + 200GB (worker)                 |   -   |       40.80 |
+| **Total**     |                                                   |       |  **188.67** |
+
+> \* Cloud NAT also incurs $0.048 per GB processed by the gateway (both ingress and egress)