SaaS recurring billing to use with my Ruby app?

I'm trying to add a SaaS recurring billing solution to my app, which is written in Ruby, we use the Sinatra framework and things like Riak.

The nice to have would be:

• A RESTful API
• Ruby API
• Returns and get JSON data (since with that will be easy to integrate with Riak)
开发者_开发问答
• Cheap (we are a startup, so no enterprise solutions)

So far I was looking at: http://chargify.com/