A 1987 Gn in 87 was $16,617.00 according to Motor Trend
http://www.motortrend.com/news/1987-buick-regal-grand-national-first-drive/
A 1987 dollar today due to inflation is about $2.10.
So a GN today would be $34,895.00
I imagine an untouched low mileage (0 - 200) would be worth a bit more...