The Afterburner appears to be $140 on GuitarHeroStore (formerly RedOctane).
GuitarHeroStore.com - Buy Afterburner? Metal Dance Pad
The AB is a great pad, I have one, I have friends with them. They make for some sexy doubles play if you feel like grabbing a second one (I actually learned to move around the pads using a pair of ABs). They are a little tougher to play hard songs on than an arcade pad is, but they're a great investment.
The only problems that I'm really hearing about them are the arrows cracking, but they still work fine.
If you're not looking for the supreme DDR luxury, the AB is pretty good, and a LOT cheaper than a Flux is.
EDIT: I just saw they have the Afterburner 2.0 now (the one Deathrazor mentioned that is $200), and it appears that the only difference is a different control box that supports Wii and Gamecube. If you want to play on PS2 or PC, just get the original Afterburner that I linked. It has connections for both, so you don't need an adapter for playing SM.