Shops owned by family members usually let you come in and mount and balance your tires yourself because the weights cost about a nickel each (as long as you're not a dick, and stay out of their way). The mount/balance cost is all labor.

If you don't have one of those... Average price is usually around $8 a tire to balance, and the same to mount. Most shops charge $5 to dispose of tires. So... uh... same as what's been said, and why did I post?