Do you use PayPal for transferring and receiving money? If you were sent 250$, do you know how much is actual amount? Or for receiving 250$, how much total amount be transferred? Paypal charges you mysterious fees, which only appear after you receive and will be deducted from your account.
For calculating the Paypal fees, there are online convertors which show the breakdown of the fees before sending or receiving money. Not only then can help with above scenarios, they can help simulate various other options like for business accounts, different geographic regions, transfers etc.
Also, if your trying to calculate the most mysterious fees of ebay before selling, Ebcalc.com can help you.