Try using this calculator to get an idea. http://ecal.altervista.org/en/fee_calculator/ebay.com.au/ 

For example, if you sell an item for $100, you'll be paying about $14.45 in eBay ($11.39) and Paypal ($3.06) fees. So about 15% of what you sell goes to eBay and Paypal. This is a very large component of a sale when you take into account any profit margin you may have. The percentage and/or fees charge will be higher if you have to pay insertion fees, if you choose add-on features which cost more and/or if you have to add postage costs to the mix.