alpine type X is only around $250 off of woofersetc.com all youd have to do is save a bit more or you could get an type R for $150 maybe a bit less those give out good SQ. YOu can also try Eclipse TI or DD2500 would be a really good bet. You would really like the dd2500 10" they go loud and sound really good i use to have a pair i had 2 10's hitting 150!!
F the type x they basically suck so much better out there. type r isnt going to have sq near the assassin. TIs and DDs are awesome subs but he says he wants something cheap so he should definatly go with the assassins
they are awesome subs youll love them give them like 400rms each