Document expected base AWS costs for running full template

[lorenyu]

Useful for budget forecasts, especially for smaller projects and teams

[KevinJBoyer]

Suggest adding this as a new page at /docs/infra/costs.md, then referencing it from the root README.md, perhaps under under Setup? ("See here for an estimate of the default costs.")

The costs.md can be simple/short:

• The expected monthly cost of installing the infrastructure template with basic defaults (e.g., one environment, and a single database), as of some particular point in time. Probably one paragraph total.
• As a bonus, maybe point folks to modifications they can make to reduce costs (e.g., "A large proportion of the monthly costs are for the custom VPC. If you don't need a custom VPC, you can modify the service and database layers to reference the default VPC in the AWS account, then remove the custom VPC."