Oh now I get it. Thanks for your help
I got curious about something... is there a fixed price list or something to know how much fee is needed for x kbytes ?
The fee is not fixed. It varies depending on the number of unconfirmed transactions. I use this site:
http://www.cointape.com/ to estimate fees. It does satoshis per byte. You just figure out the size of the transaction and then calculate the fee based on that.