The cost is a percentage, not a fixed amount, so they cannot tell you what the fee is as it is based on the selling price of the item.

There is a recommended rate depending on which category the item is in, but you can adjust it up or down, so it's entirely up to you how much you want to spend.

If you go into edit any of your listings and look after the postage section, you will see a Promotion section. For me, this comes up as about 8%, but I can change it to whatever I want. I use it for some of my listings - mainly those which are a bit older and I want to try and move the items a bit quicker.