I am "amateur professional" painter; that is I have taught myself paint and bodywork over about 20 years. My hobby, before this initial bike build, was repairing high end, late model, insurance wrecks (Porche, BMW, Lexus, Mercedes or anything I found interesting). You can see my most recent paint job here
http://www.xs400.com/forum/showthread.php?t=7760 .I think it came out pretty well even though my initial attempt using cheap paint was an abject failure. If you have access to a compressor you can buy a decent paint gun (I use a "touch up gun" for motorcycles and smaller auto parts) for under $50 and do it yourself. However, costs of just the paint/supplies can be a little offputting:
1) 1 qt Direct to metal primer and activator $50
2) 1 qt 2k high fill primer and activator $50
3) 1 qt PPG Duron base coat $70
4) 1 qt base thinner $16
5) 1 qt clear coat and activator $35
6) filler, sandpaper, glaze, tape $50
(These costs are from memory and may be a few dollars high/low but are reasonably accurate)
Additionally, as noted in my post, I have about 100 hours of labor just in paint prep and painting. Although a professional shop's costs may be lower by having several of the above items bought in bulk and/or leftover from large jobs and they may be better skilled and not need the number of hours I put in to the paint work, you can see where $300 is a bargain.
And yes, if painting outdoors expect a fair amount of dirt and dust in the paint job. However, if using a base/clear system much of that can be wet sanded out after the clear cures.