Not sure what KBB gave you for a range, but I have read often to not put too much weight on the "market value" they give. These cars have a huge "desirable" quotient on the open market, especially in top condition.
I did a quick search on Autotrader.com, and low mileage SEC's 90/91 were asking $20-$25k. That's a lot of coin for a 10+ year old car - but obviously supply and demand economics are in effect.
Wish you luck!!