PSP games have lost quite a bit of value since the PS Vita has been given a release date I'm guessing between ￡15-20 or $30-40. It does not depend on who serves you it only depends on the quality of the game and it's packaging. Lack of a manuel can affect the price given.
Also take into account trading in for cash/store credit will equal 2 different amounts (store credit will gain more).
Somewhere around $60-$100. But I would guess it would be close to $75. $7-13 per game.
you could get anywhere from 5-10 to 20-30 depends on who is working that day in most stores.