We've been using AR400 3/8inch for the last several years and it holds up very well. We're also lucky in that we have a member that owns a water jet cutting business. Cutting AR, or any steel, with a torch draws the carbon to that area and weakens the steel. The last lots of steel we had made into18x19 inch targets. He then cuts a square hole about 2-3 inches down from the top edge for a 1/2 inch carriage bolt to hang the target on brackets he also cuts for us. That hanger style lets us reverse the target face each year.
An 8ft x 8ft sheet gets 25 targets but the cost around here is $810- $840 for a sheet that size; depending on the market at the time. I just got a sheet for another club I shoot at and that size of AR was $835. They got 25 new targets which they needed badly, and the remainder of the $850 they had got the hanger brackets using 1/4 x 4 inch flat bar.