How much leg work have you done to compare the cost of new vs used tires?
My wife's mini van just hit 50K in miles and needed new tires. The first quote I got (from the dealer) was over $1100 out-the-door.
One Saturday morning of calling around and I finally agreed on a quote of $468 for the 4 tires with a better tread warranty than the original quote. (ended up buying from Costco)
BTW these were Bridgestone tires, not some off brand. The key is to call around and get the true, out-the-door cost with taxes, fees, mounting, etc.