Are you wanting to sand it down to remove the texture or just to clean it up?
Last weekend I saw an Oakie with a rear spoiler that had been sanded down to remove the texture and then sprayed the same colour as the body. Looked mint. It was at VW Sport who were taking it to Dubfreeze to sell. ?2k with a Corrado 2l 16v engine in it too.
It would take ages to sand a bumper down to a smooth finish though, probably why they charge a lot. Several hours with a power sander I'm guessing.