I think most logo work goes on a set scale, but I don't know what the price would be. I think it depends on how much collateral or auxillary materials you are designing (packaging, etc). No matter what, you've still got some years before you can charge for a logo (unless it's for that rich kid's parents, then go for it! j/k).
I was gonna also suggest in addition to the advice above, go to the library and check out some books on logos and see how all of the advice given above has been applied (or could be better applied or applied differently)...this will give you a real feel for what it is all about.
Good luck