Here is my budget round pen
Built from fence posts, bought at a farm auction for $1 a piece, and a lift of second quality 4" x 2" x 8' planks. I think the whole thing cost me less than $500 in the end
No it isn't the sturdiest in teh world, but despite some online doubts about when I first posted it has served me well, and no one (so far) has jumped or crashed out of it. The worst damage was Willow in a bitchy mood breaking two rails kicking out at Emmy who was grazing outside.