I put 400ct 3/4" solid balls in my cart and it came to $44, vs the $99 they want on the site.
They are using hollow balls, though I can't see them being more effective.
Main thing is to greatly decrease the surface of the water to minimize evaporation. The solid ones will be less effective... would hollow ones be worth 2x the cost... don't see it offhand. One may be able to find them even cheaper yet or hollow ones for the same price; that link was the 1st one I found when searching Google.