Sony's pricing of PlayStation Move is pretty clever. They can get away with saying that the Move experience costs less than $100, since the Sports Champions bundle -- which comes with the game, the PlayStation Eye camera, and the wand -- retails for $99.99. (And if you already have the camera, the wand itself is $49.99.) But technically, in order to have access to the full range of Move games, you'll want to shell out an additional $29.99 for the Navigation Controller (i.e., the Move's analog of the Wii's Nunchuk).
Thankfully, there's a cheaper solution, although it's less than ideal. PlayStation Move software engineer Anton Mikhailov affirmed at E3 that the DualShock 3 and Navigation Controller are interchangeable; you can hold the DualShock 3 in your left hand like you would the Navigation wand, so a controller that you already own will function just as well as one that costs an extra thirty bucks. (It's not confirmed that the Sixaxis controller will work, too, but I don't see any reason why it wouldn't, since the DualShock 3 is literally a Sixaxis with vibration functionality added.)
I just tried holding my DualShock 3 in my left hand alone, and I have to say, it's not all that comfortable. Then again, if I were to buy Move, I think I'd rather deal with this relatively minor issue than spend another thirty dollars. You can decide for yourself when Move launches on September 19th.