Ultimately, it depends on the value the app has for the buyer (not how much money you put into developing it). Unfortunately there are many reasons for a buyer to purchase an app and so there are many ways to value an app (a multiple of yearly revenues,number of active users, etc.) . A good solution to this problem is to post your app on an auction site such as http://www.apptopia.com/ and let the marketplace give you an answer. Good luck with your sale.