Retail is $20. It's still a great game ino but If you have to get any version, I'd go with the ps2 version. The wii version suffers from long load times, slowdown, and tacked on wii controls.
On the flip side, BT3 is fantastic and feels more like it was developed with the Wii in mind. A used copy is $26.99 at GS/EB (less with the usual discounts) and I'd reccomend picking that one up instead.