try going in the blockbuster rental shops, they sell off ex-rental discs cheap. when we were there in feb they were 3 for $30 and we've never had a problem with a disc not playing. some of the films are very recent as well.
if you want dvd's reallllllllyyyyyyyy cheap go away from the tourist area and there are smaller dvd rental places (you really have to look for them!) We managed to get 3 ex rental recent dvd's for $15.
also if you buy used discs i believe they are not vat liable so if you get stopped as customs they can't charge you vat on them - best check this out first though, don't take my word for it!!
this time we broke our record and only brought back 45 dvd's