You are probably looking at the equivalent of $100-200/hr for good development, with the higher end for people located in SF.
If you are going to out-source development, and you have a design/product background, I would build out a product spec and even a draft list of stories into Pivotal Tracker. It will help you define with your contractor what you want but also help to estimate total job time -- and that will help you arrive at a budget, based on the above hourly rate.
[If you don't have a design/product background (or are not a developer yourself) then the success of outsourced is probably going to be mixed]
I also tend to shy away from fixed bid because it doesn't work with an agile approach. You may not know completely what you want and as you get building and can get a better picture, being able to add extra components into the mix will cause friction with a fixed bid contract.
Better to agree a budget so that everyone is clear what the limits and goal posts are.