To have the design that you illustrated done totally by machine, you'll need a machinist with access to a wire feed EDM machine.
EDM rates are in the ballpark of 100/hr with one hour setup time... And probably 1-2 hours run time.
If you decide to compromise on your pattern, and would accept 1/8" minimum internal radii... Then your new pattern could be done on a CNC mill.
Millwork runs between 40 and 70 bucks an hour. You're going to have 2 setups (one for each side). The shop would probably charge you 1.5 hours setup time, and still 1-2 hours run time.
If you want, some larger shops can offer handwork to specification. That would mean having your part milled with 1/8" internal radii, and then filed to get sharp corners internally. Rates for this will very.
The top slot will be a whole 'nother setup. Has to be done on a mill, but can probably be done on a manual mill. I don't know what the guys around here charge. I think you'd be looking at another 30-50 bucks to have a shop do it.
The most important thing to consider above all is the opportunity for failure. Custom work comes with a risk. And if a shop messes it up, they might not charge you, and you certainly would not get another gun to replace it. You want a very intricate pattern done. And that increases your risk significantly.
Finally... Don't be concerned about a professional machine shop that's never handled paintball equipment. Just tell them not to scratch the bores, and you won't have any problems. Your below-average machinist has handled parts much more critical than a paintball gun.