My IT department at work receives a lot of a particular computer, shipped in ethafoam. I asked them to save the foam for me. When I had enough, I stacked it all up to the size of a hay bale, and wrapped it like crazy with pallet wrap. When it's all shot up, I just wrap it again with more wrap. It works fantastic and the target has lasted 3 years, with a lot more left to go. The only down side to it is that it is light enough for the wind to blow it around when it's gusting hard. But it takes broadheads and field points just fine. I've also shot judos and rubber blunts into it with no complications.
Last summer, I bought this spider target-thing from the local compound shop when they were clearing them out. It says, "10,000 shots" right on the side. It lasted about 2 months. My homemade FREE foam target is putting it to shame.