Generally speaking, the best way to work this out is just to test it. You'll also be able to source industry numbers fairly easily I would of thought but you can't beat real data for your particular site and audience. If you're yet to launch then obviously industry benchmarks will be more useful - but you should still be able to do some testing. Another thing you can try is call a media agency that buys and sells advertising - ask for their rate card and assume 50% of what they offer as a benchmark. I'd also make sure you forecast and test lots of different types of advertising - cost per click, cost per mille and cost per action.

It sounds like you want to get some feedback beyond just UX testing? If so, then your best bet is to find some customers - even better, some paying customers. Offer an introductory rate and keep the numbers small. The introductory rate could be free but better if you charge for it as you'll get a stronger qualification of the people giving feedback. You can get the initial group by being active where they spend time online, or build a community around their shared interests within appropriate platforms for the audience you're trying to target. Consider using paid ads as a quick way of getting the initial interest but don't promote the app directly - promote the shared interest first - if they engage with you, then they'll naturally move on to try the app. Within Facebook ads for example, you can easily target competitors / similar communities. Another way might also be to approach an existing app owner with an established community where you can naturally offer value to their audience but don't conflict or challenge what they offer - the owners/ team might also fall into the testing group and they can also tell you how they built their initial testing group before they launched...I hope this helps - cheers, Ben