Pricing for freelance jobs can be tough, and much of it is driven by the market. As I'm writing this, for example, the job market is still pretty bad, and there are a lot of people out of work. This is driving the price for freelance services lower. People are more desperate for projects, there are a lot of people willing to do the jobs, so people who are hiring can push the prices down lower. (I know, as a freelancer, you think that sucks, and it's unfair. Well, don't worry—when the economy gets better, the tables will get turned, and the freelance rates will go up considerably.)
You'll find that people hiring out for projects will often try to low-ball you, potentially even asking you to do work for free in exchange for letting you use the work in your portfolio. Now, I can't tell you what to charge or which opportunities to take or pass up. What I can tell you is to charge what you're worth. Clients suck, and the money you'll get won't just be for your work. It'll have to cover the time spent dealing with their nonsense, questions, redos, complaints, etc. It's rarely easy, and it's almost always a pain.
You can get a baseline by looking up what average freelance rates are for different services (simply do a Google search for the term “freelance rates”), but don't use these as anything more than just a guideline. My suggestion: Charge what you believe you are worth. Charge less if you're building your portfolio, more when you're better established. Negotiate, but don't be afraid to reject a project if it doesn't pay enough. If you accept too little, hiring managers will see your services as lower quality. And once you begin working with a client, it'll be very difficult for you to raise your rates with that client in any significant way.