At the moment you list a product, you see in your "Manage Inventory" how much FBA fees Amazon will keep. Then you take off the cost of your purchase and shipping to Amazon, monthly fees, storage fees, low inventory fees etc. and that gives you your profit. (From your profit you pay your internet + phone, your insurance, your LIVING).

So if you see that the benefit is not enough you have to rise your sales price. Amazon will not give you a refund.