Over the past few months I have been developing an ebay profit calculator, you can check it out at www.ezaytools.co.uk
The website allows you to calculate the 'real' profit you make on ebay - it includes all the hidden charges (such as final sale commission, paypal and so on) there is also a handy postage calculator that can give you the postage cost for a given service and can also calculate the cheapest postage method for a given weight.
It also makes recommendations to make sure your list price will make you the most profit (sometimes dropping your start price by 1p can lower your listing fee by 50p!)