Nice score! That's one of the hardest games for the Xbox to find. It is indeed a full game and given away at Volvo dealerships for a limited time. I believe there were only a few thousand of these made for distribution across North America.
I think many are under the impression that it is a demo because there is only like 3-4 cars and 2-3 stages or something like that. It's usually found for $20 or so used so you did well indeed.