I use to take 3 ft property stakes ( 1x2 ) and put two together like a pair of scissors. One set on each end with a piece of plywood about 2x3 ft spanning between them. A short string or small chain kept them from spreading too much. I drilled a hole in the top and stuck an old cleaning rod in the hole with a ribbon.
There are an awful lot of good ideas in this thread and I may look into using some of them. But so far I have made all my target frames out of scrap lumber screwed together with deck screws. Never had much problem with them getting shot up.
But the OP specified low budget, low tech options. Some of these ideas are a little more in the higher budget, high tech realm.
I use a piece of 3'X3' plastic snow fence held up by two pieces of rebar. Hold the snow fence to the rebar with zipties. Attach targets to the snow fence with clothes pins. Can't get any cheaper or more portable than that. But I don't move it and it has stood up over a year to sun, wind, and snow and keeps on going!
My original frame was the same, just scrap 2x4s screwed together with slots to hold a TGT board at the right height. I ended up going through two of them over the years as the wood rotted and/or was shot up. I definitely got my money's worth. This is it with the 50yd "extension" clamped on: