It isn't too difficult. I got a temporary front bumper (other one was destroyed due to a tow truck) and wet-sanded over it with 1000 grit sandpaper, then again with 1500 grit. Sprayed down a $3 dupli-color aresol primer, and a $3 dupli-color aresol base black, followed by a $4 dupli-color clear coat. All in all, for about $15 I painted the bumper myself with rattle cans and it came out just fine as a temporary fix...it was the bumper in the current pics in my gallery and I thought it was worth it until I got a real re-spray.
That being said, doing the prep work yourself might save you money in the short term, but I highly doubt any body shop will warrenty their paint work if they don't have control over all the steps. So though you may save $150 in prep labor, the $350 you spend on paint and painting labor might be wasted if you get less than optimal results.
BB 50-trim 6-bolt 2.0L