$400 each - from auto salvage yards.

If the filth is on the outside, I find that 1500 grit wet sandpaper followed by Mequiar's PlastX does the job nicely.

There are threads here about various methods and products used to clean them up - I wish I could find the thread that I used for instruction.... took me about an hour to finish mine.
Total cost was around $20 for PLastx and sandpaper.