The cheapest pen I've built was just some PVC piping wrapped in wire.
It was not predator proof at all, but I didn't need it to be.
I've also constructed 4 pens that I currently use out of wood and wire. (below)
Look around your barn and see what materials you have on hand.
The most expensive part of any coop or pen is the predator proofing (wire).
I built them the same height as a roll of wire, so it would be real easy to just wrap the wire all the way around and staple.