Really? You should be able to figure it out. There are too many unknowns for anyone other than you to figure it out. Are you a Pro Seller or Individual Seller on Amazon. Are you a Top Seller on eBay? Are you offered a waiver on your eBay listing fees?
Now that I think about it, why would it matter who’s the ‘cheapest’? Wouldn’t you be more interested in which platform generates more sales for your business?
Let us assume, for arguments sake the fees on Amazon are 15% and the fees on eBay are 10%. Would you care which is cheaper if on Amazon you can sell 100 units in the same amount of time you can sell 20 on eBay?
I am all for trying to reduce overhead, but to do it at the expense of revenue is just wrong. Costs of business is a factor, but weighed against sales volume. No one knows your sales numbers nor your account statuses which are key to answering your question.