badger, keep in mind that its a game and supposed to be fun for both sides. - not to mention roleplay. what will a pirate do when he is payed 500 million - buy a system and crown himself to be king?
for a person, 500k-2million is FAR more than enough. if you want more realism, you can base charge 1million and add ( roughly ) your expenses, that is - repair costs, ammo etc. whatever you used on him - added up generously.
you do not want to penelize the player - roleplay is to penelize the character. pirating is a responsible job; much more than just charging a tax. it does involve charging just enough so the trader isn t laughing ( like charging 1 million from the UN seen in austin powers by dr. evil:D) - and it should not be so much so its a real pain to the gameplay of the trader. a decission about paying or not should be within roleplay for the trader character. if you charge 500 million ( i know it was just an example ) its not a character decission anymore, but a rational player decission.
keep in mind, pirating is a quick job - trading for X amount of money takes long, - what you are getting payed with is the traders time.