Matt -
You can use Swagger to get API related info on Cost Transparency: https://<<InstanceURL>/api/index.html
For example: https://contoso.apptio.com/api/index.html#/Projects - will show you a list of accessible domains and projects.
EDIT:Please note that APIs in above link aren't generally available yet. They are under beta testing, exposed for getting feedback. While we are working on these APIs, we don't actively support them yet. Please use these at your own risk.
Best
Arpitha