Ingredients cost + labor cost + per-order overhead cost + 25%.
That's a loaded question. Depends very much on what area you're in. I'm in rural Iowa and people gasp at the thought of paying $500 for "just a cake." Around here they want something for nothing, so it's hard to charge what you're worth without hearing the complaints roll in.
Having that said, I do have a method in figuring my prices. As a rule, I figure my ingred cost first. Then I estimate how many hours it will take me and pay myself $12-15 per hour, depending on if they have chosen a really easy or really complicated design. More complicated means more per hour. Add in delivery--figure out your car's mileage per gallon to see what it will cost to drive there and back. Pay yourself for your time on the road. Add in any other expenses--cake stand rentals, special things you have to buy, etc. I also figure in if they called plenty in advance or if it's a last minute "Oh yeah, we should probably serve cake" kind of thing. Inconsiderate people who want you to throw a wedding cake together on a week's notice pay more. 
Yes, it's happened. 
A good thing to do is to check up on your competition. Then you know if you're in the ballpark. Check the internet (most independet bakers have a facebook page or website with prices posted right on it) or call the local bakeries and ask what they charge for things. Or get a friend to call for you if you feel uncomfortable doing it yourself. It's nice to stay a little bit competitive with bakeries and others, but you have to know what your time is worth too. Don't underestimate your worth. It's a common mistake to underprice your cakes because you don't think you're good enough to charge more. We've all done it and regretted it. People will value your work if you value it too.
Good luck!
I tried to find that link for the OP but had no luck. I think it should be a manditory sticky on all forums, as pricing questions are the ones that are asked the most.
Um.. I take that back. legal versus non legal could use its own sticky too! 
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