Enough to support me for a year, and potentially another developer. Plus costs of production server(s) and upkeep. This is negotiable. Shares in company also negotiable.
I assure you that I have done more research than anyone in this field-- I do this full time. The tools I need to run my business simply do not exist. I have created many of them, and the one I am working on now is the "big one" that all of my partners in the field wish they had. I have been developing on this for months, and not just on a whim.
The website I linked to can charge $50-$500/month for a reason: they have a monopoly on their 'market.' As I would with mine. The markup is near 100% on their application, judging on the demo I built that mimics their own.
I don't follow the Twitter model-- I monetize websites every day, run split testing, etc etc. Like I said, this is my job and I'm not some young kid with empty dreams.
Note on angel investors: most do not deal with web applications. The only one that does, and has sensible terms, is Y! Combinator. However, they demand that you have a team and I am running solo. (They will accept solo applications, but odds of 'winning' are dramatically less)