I can certainly appreciate your concern; that stuff costs a lot of money. Sackreet used to have a statement on their label, "If the post could be set in undisturbed earth it wouldn't need the concrete at all. The concrete is used to make a solid connection between the post and the undisturbed earth".
With that in mind, if you could put the dirt back in the hole in layer and tamp each layer to the same compaction as the undisturbed earth you wouldn't need the concrete.
I have a 20' flag pole in my front yard which sometimes has 2 3' X 5' flags on it. I set the PVC socket for that pole by compacting #56 limestone around it. That is very small rocks, not much bigger than your nail on your little finger.
The pole has stood through spring rains, winter freezes and winds up to 50 mph with rain, and it still looks good. Rocks are cheaper than concrete.