The cost to use pine pellets will depend on your supply cost and the volume you use. I have two cats and one 35 lb bag (16 kg) lasts around two months. Each bag is $7 US ($10 CAD) so it's about $42 US per year (plus tax). The bags at Tractor Supply are bigger. Avoid buying the same product packaged for this use—I've seen it for twice the cost of the ones intended for horse stalls.