We don't want to finance our vehicles. View it as a way to keep a person in perpetual debt. Like the noted scenario after you pay it off, it's worth nothing. I believe this to be most true in the low to mid range car segment. Honestly, if you don't mind a payment, leasing might be a better option.
Really, these cars are worth what people want to put into them. I'd like to keep mine to hand down to my daughter some day. Tryin to build up a pile of stock parts along the way. It's worth it to us. The effort to restore and preserve is something that's becoming antiquated. I typically go against the grain, or maybe just call me oldschool, but keeping the car on the road brings a true sense of pride as well as keeps my time occupied.
Grand Prix's are almost aged out of the "book value" scenario anyway ..unless you're trying to sell that GXP you bought this season, now it has a trans issue and you still got payments. Then you're screwed, lol.