I found a site (through several hours of research) that appears to have the absolute cheapest prices on name brand ATV tires.
Just an example -
Dennis kirk site - Kenda Bear claw 25x10x12 = $71.99
BC Offroad same tire $49.99
They offer free shipping (to lower 48) on 4 tire orders too. I know Dennis Kirk isn't typically the cheapest, but they have a "beat it by a buck" offer which claims they will beat any advertised price on some items by $1. I haven't played these two sites against each other yet but it may be interesting.