Short answer - they are rare, and people want them. Supply and demand.
Long answer, they are good cars, have been in numerous movies, are well known for their tuning potential, bulletproof motors, high power output, rear wheel drive, bullet proof drivetrain, last great toyota until the FT-86 comes out.
www.allcarwiki.com/wiki/Toyota_FT-86 - that will be impressive, it's not quite a replacement for the Supra, however, it is comparable to the NA supra.