About £100-£150 at a reputable Bodyshop.
BodystyleUk tried to charge me £250 for 'prep, spray and fit' for my spoiler, but i knew a few people that had just had a LOT of work done there so i got it for about half that. You get what you pay for. You can prob get it done for as little as £50, but you never know how good the finished job will be....