The main cost is hiring an accountant to do the tax return for you. Or figuring out how to do it yourself using software like TaxTron.
Not that I have anything against accountants, but what kind of effort are we talking for filing the taxes yourself? 5 hours vs 1 for a personal income tax return? 10 hours? A weekend? How many pages are there for a corporation (in the simplest case -- I'm sure there is quite a range)?