This is what I spent:
4mm plywood- $7
1mm sheet of plastic- $5
2 pieces of flat trim - $2
Grand total (opens calculator app)- $14
I priced glass first, but plastic was cheaper. It's easy to cut this thickness of plastic with a box cutter and ruler if you need to cut it down.
The size of the 4mm plywood was 1820X910. I cut it into 4 pieces about 880X450, so I could build more frames of equal size in the future.