I'm currently working with a freelance Rails dev on an hourly basis to build my MVP and the thing that bothers me most is not knowing how much time it will take to get to the point I want to be. I have every screen mocked up in Illustrator and most of the pages coded in HTML/CSS.

Depending on how much work had to go into the app, I'd be willing to pay $5,000 or more for a service like this. The challenge would be putting together a structure that allows for payment flexibility according to how complex the project is; comparing any two projects side-by-side, the amount of work required would vary greatly and it'd be hard to be able to throw out a universal price for everyone.

Check out http://ibuildmvps.com/ , it looks like this guy is doing exactly what you're proposing.

If you're looking for some work get in touch, I'd be interested in hearing more about what sorts of projects you'd like to work on and what your capabilities are. Email's in the profile.