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 per month and lets you manage up to 3 APIs, and an Enterprise plan which can be tailored to fit your needs; pricing for the Enterprise plan isn't mentioned on their website.
Apiary on the other hand has one Free plan and two paid plans - Standard (pricing starts at $99 per month for 10 users) and Enterprise (pricing isn't mentioned on their website). Some key features of Apiary are github integration, creation of a mock server to try out your APIs as you design them and RSS feeds.
You can check out a detailed comparison between the two products here http://www.analyzo.com/compare/156/1,2,5,6,7,