Have you looked at Customer Development?
So you have an idea for an app. Customer Development tells you what you need to do to see if the app actually has a market & what the market is worth.
Things you need to do - and you can do all these without having the app ready, or even the mockups (although mockups are needed for 2nd half of the process)
1. Do the first approximation business model (http://www.businessmodelgeneration.com/downloads/business_mo...) Learn how at ^1
2. Get out of the building. Talk to prospective customers IRL. Iterate and define, product value prop, market segment, channels. From this identify how big the market is & how much it'll cost you to reach this market. Rule of thumb - for every 100 customers you need a year from now, talk to 10. If you're B2B and need a hundred customers, make sure you talk to 10. If you're B2C and need a thousand customers, try to talk to a hundred.
3. If the market is big enough and possible to reach, you now know how much you can afford to spend. Bonus, you should now have a list of beta testers who will be interested in trying this out.
4. Build minimum v0.1 of the product or all mockups. Validate/Test. Beta roll out. See traction as expected.
5. Raise angel money to build v0.2. What angels would like to see - clear identification of large enough market, clear validated value prop by talking to enough prospective customers, mockups or preferably initial version.
6. Build product & Win!
^1 To learn how to build the business model, use Steve Blank's classes - http://en.wordpress.com/tag/lean-launchpad/