Without seeing the car and knowing state of how it runs, it could be easily worth $9K-$13K. Bumper fillers are a couple of hundred dollars and at 20 yrs old they deteriorate. I am not sure how much you are trying to get towards a downpayment but the Turbo Regals will not go down in value anytime soon given there is a limited number of vehicles left. If it is a WE-4, the value is higher. There were only about 1500 made. They were a Regal with the LC2 engine and a black out package. Very desirable! Since it is white it is not a WE-4. If you are trying to get the car to to sell it then you may want to look at some of the Buick boards to see what they are going for. Turbobuick.com, Turbobuicks.com, T6P.com, GNTTYPE.org.
If you place an ad at these sites then the guys inquiring will have very specific questions about the car and they know what they are looking for but they know the value too so if it is a car in good condition and tune you are just preaching to the choir.
BTW - why trade the Turbo Regal for a Vette. Mine is quicker than those, has A/C and gets 23mpg (unless I am high under boost). Can't take one a road course though
I can get you more specific info if you need, but need more specific info on the car would help too. ~Remember all Grand National are stock :burn: