Honestly, unless you are looking for something small and portable, it almost makes a lot more sense to get a bigger drive.
Check
Dell Day of deals - Yesterday they had a 3TB drive for $109, and they are bound to have something similar over the remaining 10 days of the sale (nothing today though)
From memory express, a 500GB drive would be at least $40 and an enclosure would be in the $20-$30 range.