Ask HN: how to price your software contracting services?

Right now I'm working as a software contractor with a few other people. I've had some experiences where the client sets the price for the project completion, and I just deliver, and other more vague agreements where neither party is happy because I get underpaid somehow, end up working many extra hours because the client never really knew what he wanted, etc. So now I've made the rule that I always need to make a proper proposal and then contract for clients, to explain explicitly what will get done, and for how much. However, I have NO IDEA how much to charge for specific services. Right now I'm trying to help a magazine put their content online and store user comments... Simple stuff like that. Can I bill my general services at something like $30/hour? How about the graphic design component, would that cost more or less? Another example. Say I am building a mobile phone app (Android) for a client. Again, I have no idea how to price these... Before I've just gone for a fixed rate for the whole project to be completed, but that's never worked out to more than like $20/hour, which I feel is pretty low. So I could use some general advice with how to price my services, and maybe get some ideas what others are doing. The best way to describe my current position would be a freelancer who's trying to start a small development firm. I'm relatively young and experienced I'm guessing though, since I'm still in college. Thanks.