I'm building an API on top of MTurk, http://houdinihq.com, that you could use. It's currently in alpha, but feel free to email me at presto@houdinihq.com if you're interested in an api key.
How do you plan to accept payments? How much would Houdini itself cost (per task, per month?)? Do you plan to accept non-US users?
Pricing is still TBD. MTurk charges will be passed through directly to you with Houdini-specific charges either per task or per month.
Yeah, I'm planning on accepting non-US users.