depends:
how long he's had that equipment, dual/single voice coil subs, how old the models are, check with meter if everything is working, what kind of box the subs are in, sealed/bandpass/etc... , new/old head unit....
based on what you have told us: $300-$500
hint: never by a car based on extras.. check online blue book for trade in value and try to get it as close as you can to that
