Selecting and (especially) applying paint films and finishes is a professional job. A large scale shop or manufacturer would, all things being equal, have a great advantage in experience in both areas.

There are typically two kinds of finish; paint or conversion varnish.

Paint is applied with a brush and when a contractor is involved its subbed out to a house painter. This is the cheap way of doing it. Painted surfaces will only hold up only so long to moisture and chemicals.

Conversion Varnish is a two part industrial finish that is applied with finishing system. Its basically the best finish possible for moisture and chemicals. The technology and skills used to spray cars are basically the same needed to spray-finish custom cabinets. Conversion varnish can be tinted to any color under the sun, just provide a Benjamin Moore # ( or similar ).

It takes a certain level of skill and equipment to be a finisher, there is a lot of prep work and between coat sandings. Trust me the house painter that the gc hires wont come even close.

I have been made despondent by my some of my clients decisions to "save" money and have "Jerry" the guy who is painting the outside of my house also do the kitchen cabinets. Inevitably, I come back to take photos for our portfolio and I take one look and the deep brush marks, hairs in the finish, runs and flooded inside corners. I don't even unpack my camera, I just shake my head and walk out.

"Paint grade" can mean: maple not suitable for stain, MDF, poplar, could mean alder though not usually from a large mfg.
Factory appiled paint usually has a catalyzed top coat. This will be tougher than most of what you can get locally. The down side to that though is if it chips it is more difficult to touch up or to repaint.
Site painted are less durable and will eventually need repainting. But are easier to repaint. The best are done by a professional.
Price difference can be all over the map. IMO, at least in these parts the biggest difference in price has to do with construction. And I don't mean drawer glides, dovetails or plywood thickness. The biggest price (and quality) difference is in frame construction.
Pocket hole frames will be the least expensive and account for more than half of all cabinet sales. Again around here that is the most common with local "custom" makers.
Next up the scale is a doweled face frame (sometimes loose tenons or biscuits) and finally the best and most expensive will be true mortise and tenon. In mfg brands that will be in higher end and true custom mfg's
You will also find variations in the material used for the sides of the case. Particle, ply, varied thickness of ply, and source/quality of ply. (Chinese, mexican, canadian domestic, NAUF, and a myriad of grades)
All of those contribute to price difference as does mark up.
So you decide how you want to deal with paint. Talk with a few makers/dealers, find someone you like and trust that can make your budget.
Most (not all) mid price and up options out there are "good" nowadays.

Fantastic information from all! Thank you, Clarion, Cabmanct and jakuvall. Exactly what I needed to know!
:-)