I recently tried the Rustoleum Re-Color that is advertised on those late night infomercials and found that to be quite good at returning the luster to dull plastic. I was getting ready to sell my ATV, and after years of riding in the PA woods here, the plastics were faded and had no shine at all. At $20 a bottle (and its a small bottle!), it too isn't cheap, but I was able to do the entire ATV from top to bottom, and it looked as good as the day I bought it.
The only downside, and its not RREALY a downside, is .. the smell !! it smells like they attempted to camouflage the chemical smell of the product with some fragrant add-in that really burns your nostrils. If you DO use the product, apply it outside (out of direct sunlight) and make sure it does not get wet. Then leave it outside for at least 4 hours. The smell will remain on there for a good week before it dissipates.