Took my car to get an estimate on my loose/misaligned bumper and they found the inside filler stuff to be mushed and broken up, most likely from the the previous owner of the car getting rear ended (it was like that when I bought it). How much is that stuff and how hard is it to replace? Probably just have to take off the bumper and that's it, right?

ya, are you are talking about the foam, or the brackets, ect? the foam filler piece is pretty cheap and you could take off your rear bumper pretty easy.