Here there are basically 2 schools of thought; proffessionally sprayed or DIY.
For the proffessional stuff, the most common options are Line-X and Rhino. I have Rhino in mine, and it's a bit softer (more rubbery) than Line X (all my bumpers and rocker guards are line-x'ed).
For DIY, I have heard good things about herculiner, but I have not used it myself. I did read a VERY funny post about a guy who gut herculiner on his private parts, though...
As for how well it workds, to give you an idea, the rhino is about 60~70% as good at reducing noise and heat as my stock carpet was. I still run the carpet for everyday use, but I take it off for wheeling (now I dont have to worry about the bed getting scratched and rusted).
For the proffessional stuff, I paid $400 here in Costa Rica, but it was about $650 when I was in Miami. Even Rhino has different compounds, and some are more rubber like than others, but the more rubber like you go, the less lasting they tend to be (of course, the more rubber like they are, the better at insulating heat and noise, so you WILL need to compromise here). Something to keep in mind when you do yours.