If you could do it yourself, why would anybody bother to pay? Professionals have access to more sophisticated paints that will produce a much more durable finish than anything you can buy and apply yourself.

FWIW, I think I paid $150 to have the frame and fork I built powder coated. That included bead-blasting, masking+plugging, and everything else. $150 was the minimum charge from my powder coater; I probably could have had two frames done for that price! If you want fancy powder (metallics, translucents, etc) expect to pay extra. In my area, a good wet paint job costs $350-400 for a basic job. If you want multiple colors or require fancy masking, the price goes up quite a bit.