I'm no painter, but I'm going to guess no more than $250?
My 400R front bar, for example, was going to be $400... but that included re-fiberglassing the cracks.
Then again.. the places I got quotes at wanted $200 per side to paint/fit R33 GTR pods (and they're small!)
All you can really do is go to a few places, and get quotes.. and go with whoever you think is best. You may have to actually go to the panel shops too, as they won't know exactly what they're painting over the phone. But, most spoilers are very similar in size anyway.