The easiest and cheapest is snow. The only downside is it only works in winter.
I learned this once when I went shooting a .308 Win at a plastic 20L bucket. It was winter, and I had a whole long, flat field covered in several cm of light, fresh powder snow. I filled the bucket with snow, and set it out a ways from the edge of the field, and shot it several times.
When I went to check the bucket, I noticed I could see the troughs from the bullets skimming along the top of the snow behind the target. I decided to follow them. What I found was that the bullets skipped along like they were on water. They would burrow through the snow for several meters, lift up a bit for a while, then drop back down again. I don't remember now how far they went, a couple of hundred metres total, maybe? At the end, I found the bullets, perfectly mushroomed and lying on top of the snow, or melted down a couple of cm in readily visible holes. Best bullets I ever recovered.
I have also gone back to sites in summer time where I had been shooting in winter, and found 7.62 FMJs just lying in a pile in front of the backstops. The backstops had a tendency to develop deep snowbanks in front of them, and I don't think the bullets ever hit the hard dirt.