How soon does your app have to be launched? Is there a specific date by which the app must go to market, or can you afford to be a little more patient?
If you require a very fast release and rapid response to any issues (bugs, feature changes, etc.) thereafter, then it's likely whoever you work with will have to assign one, or even a team of developers to work solely on your product. Realistically, this would mean paying full-time development rates for weeks or even months of work.
Alternatively, if the deadline isn't as important, a lower price may be negotiable.
Decide how quickly you need your product to launch.