Not all steel is the same. If you are shooting just handguns, you can get away with normal steel plate, but if you are shooting at it with small, fast calibers like the 5.56, then you will need AR500 hardened steel. It is much more expensive, and one of the problems with it is that if one heats it up to cut it, it loses it's hardness. It needs to be cut with a water jet or some other cooler process, rather than just torched or plasma cut.
That adds to the cost significantly, because that equipment is pricey.
As far as where to buy, there are a couple of sellers on fleabay that I have had decent luck with on buying AR500/AR550 steel targets from. typically, 2/3rds torso IPSC targets can be found for 2 for 100 bucks, which isn't bad.
As far as mounting them, you can go as simple as a round gong hung on a shepherd's hook, which will give it some movement and allow it to absorb some of the energy. Or you can build a frame and hang it from chains, that let's it swing. Some people just mount them to a post, which is probably hardest on the steel because it takes the full brunt of the shot without being able to move.
Also, with a 5.56, you need to be better than 100 yards away to protect the metal from being pitted, unless you have the target angled towards the ground where it will deflect the shot downwards, thus prevent the divots that would occur should you should straight into it.
So, short answer is no, you aren't going to find anything cheap that will last, especially pre-made stuff. The cheapest option is to order the AR500 targets you want, then make stands to work with them. And after time, effort and materials, sometimes that isn't worth it.
I've built most of our targets, with the exception of the six poppers we have. I don't like to think about the money I have tied up in just steel, but I do enjoy shooting at them all the time.
