A lot of guys put truck bed liner in there (the kind that paints on). It seals and protects against rust, is very hard like epoxy so it won't scratch off like normal paint, and is textured so your boots won't slip around like bare metal. Rubberized undercoating would probably achieve the same results but I'd imagine that being a little less grippy and probably costs a little more too.
You could always buy a rubber/vinyl floor cover like came stock in a lot of trucks, pretty sure you can get them from LMC and other places for around $200 iirc (been a while since I had a pickup, only currently have the SUV catalog. Or you could always just throw a cheap carpet kit in there. Even though it's not a show truck, there's nothing wrong with carpet even in a work truck (it will still look nice especially if you won't mind when it gets dirty, anything is better than bare metal). It will also help insulate against noise and whatnot. Costs more than just spraying something in there, but realistically isn't that expensive.