Paper is probably your cheapest option. However I think aspen is a good medium for your problem. It’s one of the cheapest substrates. However aspen will smell a lot stronger than coco husk. This can be good and bad. The smell helps indicate that you’ll either need to spot clean or deep clean, but the bad part is that it doesn’t smell all that pleasant. As well with aspen, I’d recommend replacing the substrate completely every month maximum. Aspen can Mold a lot sooner than coco husk. But as long as you are cleaning that way you should, aspen works great