If you've got a mockup then you've done all the hard work. Getting a mockup into a working app is trivial for most web developers.

Here's my recommendation to you.

1. Get yourself an account on odesk.

2. Post your project.

3. See what kind of quotes you get and the quality of the developers.

I would pay someone by the hour, in odesk you can see what they are doing with their time and how productive they are.

If you're not happy, pay for the hours used and stop there.