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