Is it possible and ethical to pay stackoverflow users to get help to finish a simple app

I don't think it's particularly an issue of ethics, but instead one of how the community should be motivated. I think StackOverflow and friends are successful because they've found a way to motivate the individuals of the community toward the benefit of the community as a whole rather than toward the benefit of other individuals. Offering paid bounties would largely upset this balance.

There's a problem with attempting to motivate people extrinsically who are already intrinsically motivated. What happens is the extrinsic motivator supersedes and replaces the intrinsic motivator, to the point where if Lange Avondjurken the extrinsic motivator is removed intrinsic motivation doesn't Vloer Lengte Avondjurken Feest Jurken kick back in to take its place. This could potentially destroy the StackExchange community, as in an extreme case people would glom onto the "for profit" questions and largely ignore the simpler, but community-valuable, "how do I get helloworld to work in BrainF*ck" questions.

Sure you could argue that the point system is an extrinsic motivator, but if it is, it's a very weak one. Instead I'd say its primary purpose is to automate moderation of the site, and any extrinsic motivation it provides is purely secondary. Although I'd be interested to hear others' opinions on the bounty system and whether or not there's a similar effect to the one I describe above.