I make my living in patents and have seen this story many times. I hate to say it, but the answer is figure out the business and then file the application (unless you are into lottery tickets). It is the business that makes money and you need to know how that will happen.

If you do a quick and cheap provisional filing, (a) you get quick and cheap and (b) you start the one year clock ticking. May as well put your back into it right away to know why you should spend the money on patents before you spend it.

That is my two cents.

Disclaimer: 1) I am a full time patent lawyer; 2) I have sold IP portfolios for many millions of dollars and 3) I have built multi-million dollar businesses and the third is my personal preference.