Ok, so I have been using a 50-50 mix of pine and aspen(to cut down on the dust from using straight pine) and I have not had a single problem. Those giant bags from petco usually last a few months and are only like $10. It expands alot when fluffed... It just seems that the pelleted stuff (like equine pine) would be so dense that a 40lb bag really wouldnt be too big. Do you change it less frequently, use less than if using aspen/pine, or a little of both? That is the only way that it would make sense as being cheaper to me. I definately want to try it out though, especially if it helps reduce the odor from the rats and is potentially cheaper.