The Mashape API Marketplace helps developers discover APIs for their apps. The Mashape Billing Add-on Tool allows API providers to add a payment layer to their API in seconds. Their integrated API documentation and test console let's you experience an API without commitment.
Both 3scale and Apiary are great tools for managing APIs. 3scale has some great features to offer such as log tracking, single sign-on, custom branding, analytics etc. 3scale has two plans - a Pro plan that is priced at $750 perAPI Management Services