As others have mentioned, you can already do this on Yahoo Finance for free. Notifo also allows you to set up free stock price alerts: http://notifo.com/user/stockalerts
It's certainly possible for you to do a better job than those services, but if you want me to pay, your site needs to explain exactly why you're better than them.
Correct me if I'm wrong, but my understanding is that it only serves iphone users (blackberry/android pending).
I suspect that iPhone (and generally smart phone) users will be less likely customers though, because its generally easier for them to check prices from their phone than for mobile users like me (no smart phone).
I certainly can't expect to be able to serve everyone who wants stock information. But I was right in my target market, so I'm crossing my fingers that there are others as well.
It's certainly possible for you to do a better job than those services, but if you want me to pay, your site needs to explain exactly why you're better than them.