Large wood chips! Ok.
I imagine those might be even cheaper since not many folks are after them.
Thanks!
Often free, or available for the cost of gas, if your area has a surplus of them. In my neck of the woods, tree companies can't get rid of chips fast enough and have to pay to dump them, so many are happy to drop them off for free. You can try chipdrop.com to see if it can connect you to local companies looking for a drop off spot.