I've had good results with big cardboard boxes stuffed this way:
FRONT 2/3: wadded-up crumpled newspaper (packed as tight as you can stuff it in, by hand.)
or, front 1/2 of box filled with loose scrap cloth. Old clothes, bleach-stained towels and torn bed sheets, etc.
BACK half or 1/3 of box: Bundled, stacked paper. Or scrap wood. OSB board is cheap and easy to cut to fit box.
Vary the location of your targets on this backstop. It will last longer that way.
Change paper wadding every 1000 shots.
Shake-out and reposition cloth / rags after 5000 hits.
Stacked / bundled psper or wood will last just about forever. It shouldn't even take any hits, unless somebody tries a .22 on it.
I've used something like this before. Stuffed front half of cardboard box with rags and used old magazines/catalogs for the back half. Pellets never made it past the rags. Pellet removal is as simple as shaking out the rags. Hard to get much cheaper than that.