it's like 50 bucks shipped for the chip (if u can put one in yerself) and if you want the 120gig hdd (trust me, u want it) that's about 90 bucks. the big hdd isn't a MUST though....xbox games and N64 games are the only things that rlly take up space. Premodded systems run a bit more....quite a bit I believe.
Either way, it's worth it. I have colecovision, intellivision, nes, gg/sms, gba, snes, genesis, N64, playstation, and sum other I believe emulated on my box. it's a gamers dream come true. PS1 games run really perfect and A LOT of the N64 games run real good. but ya, definatly get one....it's great.