As a freelance writer, I understand your dilemma! Do you charge for time, or expertise? By item (in my case, per word) or by job? Or just stick your finger in the air and guess?
Personally, I start by judging the time a given job will take. This is a little tricky when you first start out, but I work on the principle that I have a minimum wage per hour. Then, I work out how much a given task (say, 500 words) takes me to do. Then, allow thinking time (!), research time and preparation time. Also bear in mind you can't work flat out for hours at a time without a break. This gives you a ballpark figure for any given task within a time frame.
Now, add in any associate costs of materials, travel, etc. Then, add approx 15% 'company running costs', i.e. how much you have to spend on your business regardless of whether the work happens or not - broadband, insurance, car tax, accountants fees, stationery, marketing, etc.

Now add your required profit margin without being greedy - start at say 30% and see how it looks. Allow a little slack in order to offer a potential discount if you are asked. If this still gives you a figure that pays the rent, feeds the family and leaves a little for a rainy day, fantastic. If you can earn more stacking shelves, recalculate.

Whatever you do, don't undersell yourself. As the advert says, you're worth it! After all, a service that is free is also worth nothing....

Good luck!