The target stand frames at my club are made of 2x4's with cardboard target backers. They work well, and are cheap enough to dispose of when shot out (new backers can be replaced for a while).
The bases are made of pieces of angle iron welded in an H. The pockets/sockets for the 2x4 legs are made from 2"x4" square tubing (less than 6") 3/16" or so walls. That makes them just larger than the 1.5" x 3.5" 2x4's. Works pretty well, easy enough to move, usually wind resistant enough (tall side berm walls on the shorter ranges).
On longer ranges we planted sections of PVC in the ground (I don't know the size) and changed the dimensions of the frames (long ranges usually also need larger targets).
For work bonds some guys make new stands. But they try to cheat by using 2x2's. They are not 1.5" square, but more like 1.25" square, so they really wobble (they double up the bottom of the legs).
Lowes Depot usually has scrap/junk 2x4's for not much.