Best Buy's website doesn't list Spyro's Adventure anymore so I can't say if it's the same everywhere, but I went at my local Best Buy today and there it was, brand new, $20. I didn't plan to buy it, but for such a price I couldn't say no. I've been playing for 2 hours now and it's really great, quite different from the console versions! Console are more dungeon crawlers, while 3DS is more of a platformer.
Anyway, it might be worth to take a look at your local BB