I had a spare 250GB HDD, and I got an external enclosure for $40 off NewEgg. That's connected to my PS3 now, and I got a 500GB Maxtor OneTouch (USB/FW400) from Staples today for less than $100.
(To all the people who say FW > ALL, I'm not seeing a speed difference between FW400 and USB, like at all)