You have to shop/compare price per pound or price per gallon of foam; Otherwise, you're trying to compare the price of eggs to watermelons. Used tires will take 10-15% more foam than a new tire. Some places won't even fill a used tire while some may require you buy a tube and charge you to install the tube before filling with foam. There's too many variables to compare prices from a 100 different tractor owners.
Most tractor manuals have the added weight of any given size tire that's filled with water. The chart usually has the tire sizes for your tractor. It normally gives the gallons as well. Since the foam and water are pretty close to the same weight per gallon, use the charts to determine if they're bending you over a barrel or giving you a square deal.
Ask them to price your tire size(s) then ask how many pounds of fill that price is based on. Some simple math will tell you the price per pound and whether it's close to the pounds of water held in your tires. (All charts are based on new tires.)