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.

Is it on a downward trend? What are the ongoing costs? This will help determine a break even price.

My first recommendation would be to go back and figure out how much you have spent on development, design, testing, services, fees, etc. No serious buyer is going to be interested in your app unless you can hand over a P&L statement giving accurate numbers for net profits. I put my own portfolio on the market back in April, and the first thing the broker asked for was the P&L statement for the last 12 months. If you've spent $87K and made $99K then most buyers will balk. But if you've spent $12K and made $99K then you've got a great profit margin, and even if the app has only made ~$1200/month for the last 12 months, it still has value because it will be inexpensive to maintain. Generally, you're looking at a valuation of 1.5-2x, as Vlad mentioned. And I agree with Ian that a large userbase or steady stream of installs will be enticing to a buyer even if your ARPU is low at the moment. Unless the app is ripe for reskinning (to Vlad's point) and you can convince a shrew businessperson that this garden is poised for exponential growth and it only needs more water, pruning, weeding, and fertilizer, then I'd say you're looking at $20K max. Apptopia isn't a bad place to start, though I'd recommend against spending the $140 for the "extra" exposure package.

Cheers,
Austin

Actually, the standard is past (4 months' revenue) x 9 but lately it has been going down.
However, your mainly selling your brand and users therefore it should never just a math problem.
Let me know if you want to further discuss with more details specific to your case.