I charge what the market will bear, and that depends on what the job is, what skills and skill levels it requires and who the customer is. My rate also varies according to whether I want the job or not, and there are some customers I won't work for no matter what the rate is.
Establishing what the market will bear is the tricky bit. Overprice, and you not only lose customers but get a reputation for being expensive, or worse yet, a rip-off artist. On the other hand, undercharge and people take you for granted and don't respect you. Some people will also assume that if you're too cheap, you obviously can't be any good or you'd be able to charge more, so they'll go elsewhere anyway because they want a competent job. To be honest, I'm rather like that myself .... I tend to assume if it looks to be to be true, it probably is.
Another major factor in determining market rate is whether you are trying to build a business, or whether you're established with a steady (or better yet, excessive) workload without having to seek to drum up new business.
Trickiest, perhaps, is dealing with aggrieved customers. There is a considerable degree of people skill involved in calming an angry person down and making them see things rationally, without getting walked all over in the process.
As for my actual rates, I don't do PC work so it isn't relevant. The principes are broadly the same, though.