Thanks! We also have some more advanced apis in the works, like asking multiple workers to do the same work and automatically determining the 'true' answer. From your point of view, they'll be just as easy to use though.
Pricing is still TBD. MTurk charges will be passed through directly to you with Houdini-specific charges either per task or per month.
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?