Adding to what panzeriti said, you might also want to look into where you're providing your services. Different countries may have different rates. There might also be other details like taxes etc. For example in Australia you'll have to charge them the GST rate on top of your service fees otherwise you'll have to pay for it yourself.
The same probably applies for ownership of code and other legal details. It differs per country, so if you're looking for jobs outside your country make sure to do some research on their laws.
The most important thing for me to consider with these jobs is the reputation of the company I'm providing services to. Unless I was hired by someone I know personally I wouldn't really accept a job from a random user at some website. This is from my experience as a web freelancer. Most of my stuff for iPhone I keep for myself