Wheel weights are sold in sets of two, one for each rear wheel, so the cost you see on Sears.com would be for two. They are recommended for use with snow blades due to the low traction conditions typical in winter. Traction in summer with a dozer blade is much better so you might not need to run weights on the wheels.
Wheel weights do add load to the transmission and drive belts and reduce your overall power, so I would suggest you try dozing without them first and see how it goes. If traction is an issue, the wheel weights can be used on gravel but not Tire Chains. Tire Chains should only be used for snow and ice and could break in other conditions causing a lot of damage.