ide say 80 quid max, my mate got his roof sanded and sprayed and same with both of his mirrors for 80 quid.
personally i wouldnt sand it down myself, the trick to geting a good paint coat is in the preperation (i.e. sanding ect.) most good places even if you have sanded it down yourself will still spend time sanding it down to get it to how they want/as good preperation finish as they can so its pointless you doing it imo as your just wasting your time, and as said your only going to save a small amount of money if you sand it down to a perfect finish so they only have to spray it.