This is a little hard to answer because it is so vague. It depends on the area, the market and the strength of innovation. I know that The App Guy has a terrific podcast at http://www.theappguy.co/ and is also trying to organize a community for App developers to sell their ideas. Let me know if I can be of further assistance to discuss patentability in terms of its value to getting a sale or license. What ever you do, don't spend money filing a full patent, just a provisional. Good luck.
For the most part an idea is a very small part of creating a successful app. Sure, you have to have a good idea to begin with, but many great ideas have failed in this competitive landscape. Regarding patents, that's the last thing you should be worried about. App creators and companies do not get patents. Patents are very expensive and can take a couple years to get, which doesn't work with apps at all.
You will not be able to sell your app idea, either. Everyone has ideas. It's the ones that follow through and build out the idea into a viable business plan that have a chance for success.
Hope this insight helps you. I see way too many people throw their money away by not understanding how the app ecosystem works.
You can't sell an idea unless you are in the advertising agency business. Patents are a 5 year, $20,000 process, where at the end you might be able to license it. If you think you have a really powerful idea, you need to make it into an MVP (Minimum Viable Product). Go to few hackathons and recruit a crew.
