Used prices are super high right now. I bought a 2005 500cc Honda (or 2006, I forget) three years ago and paid $3200 with less than 500 miles on it. the same quad right now with a ton more miles is selling in the $4500 dollar range due to low inventory and high prices of new ones.
I will agree with what has already been said....don't by cheap off brands, you will regret it. Honda, Yamaha, can-am, suzuki, etc....name brands are the only way to go. My son is 12 and he handles the 500 cc just fine. I would not go smaller than a 350, though I have been on many Honda 2wd 250's that have plenty of low end grunt to move my fat butt around without a problem.

They are just super small and most will outgrow them super fast.
Good luck finding a good machine at a reasonable price. If you see one, run....and I mean RUNNNNN... as fast as you can to buy it. It won't last long.