Run a search, your question is one of, if not THE most popular question asked on this site. Also check out the business section.
Google "cake central what to charge" The first 6 links are different threads on the issue.
Google 'cakeboss cake pricing software' and check out their FAQs on their website.
In summary NOBODY can tell you what to charge. You have to figure out your own costs, your overhead etc and come up with the prices you think are fair for your work. A little market research never hurt anyone, and of you're serious about making it a business, then a business plan is in order.