I buy one 4' x 8' sheet and cut it into quarters. I put up one of the 2' x 4' quarters and cover it with one of those paper, human sillhouette targets. Four staples - on in each corner.
The middle of the chest and the head get shot out, and that leaves a big hole in that part of the plywood. But it doesn't matter. The next target covers it up and there is still a place to staple the new target at the corners. I use up about one 2' x 4' quarter a year like that.