I got the Plastics done as a job lot along with the bumper being replaced and the Intercooler being changed too so the total cost was a bit sick enducing. If you already have the parts though, I can imagine just prep and spray shouldn't cost more than £20 a peice. Will always be cheaper to get it done all at once.
I've seen quite a few chaps on here have done their own painting but i'm not confident enough for that at the moment.