$20K is a lot for "just paint", though there are extremely pricey paints. Most likely, there was a lot of body work and even rust repair. Looks like the engine was out to paint the engine bay. I would expect most shops would price by labor hours and materials and the owner approves each step, which would thus be "fair price" since agreed by all.
If you want to go cheap, search for the Moparts thread "Paint yer car for $50" (or such) by a guy "67ChargerYeeHaw" (or such). It was said to be the most read post on the internet years ago. Ran to ~400 replies with photos of other's home efforts. Could no longer find it in a search a year ago, so if anyone does, please link. Most there used thinned-down Rustoleum w/ roll & tip method. I used that to paint a camping trailer w/ flat aluminum panels, using InterLux Brightsides "boat paint", single-stage (no hardener) solid-color polyurethane w/ uV protectants. Came out smooth and glossy w/ almost no runs even on vertical panels, other than at corners where the paint squashes out of the roller, but a car body has many such curves so trickier. I used Brightsides on my 1965 Dart, mixing a little yellow with white. I had done the interior and engine bay earlier, which came out OK for those areas. I sprayed in the trunk and rolled elsewhere. Wasn't perfect when I later sprayed the outside. I tried an electric rattle sprayer, but gave problems (sprayer leaked) so changed to an HVLP. Same month I sprayed 2 vehicles with normal 2-part polyurethane and HVLP sprayer while I had a paint booth rigged in a carport w/ tarps and box fan w/ house filter. The auto paint was mainly because I had to match existing new-paint on half of one car (swapped panels after crash). Took a sunroof in and they matched the color perfectly (modern grey-blue). Since left-over paint, I did our old minivan w/ failed clearcoat.
All came out about the same with significant orange-peel and a few runs on vertical panels. I wet-sanded all down w/ 1000 grit, even 600 grit first on some w/ deeper valleys, until no "shiny spots" left, which means perfectly flat. I attacked the runs the next day while still soft, cutting the end drop off w/ a razor, then 600-grit w/ small block until flat. Also wet-sand while still soft to make the job quicker. I sanded thru to primer a few places, plus found a few places with primer showing thru after wet-sanding, so added paint there w/ tiny air-brush sprayer. Can't fix those if basecoat-clearcoat paint, nor the later chips which will surely happen. Kind of amazing how once it is all sanded dull to 1000-grit, then 2000-grit, passes with high-cut compound w/ DA buffer, then finer grades quickly make it mirror-smooth.
If I ever paint another vehicle, I will only use 2-part automotive paint (w/ reducer and hardener). While the 1-part (air-dry) laid down similar (neither great) and was a bit easier to sand, it seemed much softer. Even a week later, you can smell it as you wet-sand will come off if you wipe with Paint Thinner (or Lacquer Thinner). Will probably last well since it works on boats in sunlight, and lasted well on my trailer, but harder is better for a car, since people are more fussy over scratches than a boat or trailer. The 2-part auto paint wasn't much more expensive (~$250 for 1 gal w/ reducer & hardener). I definitely wouldn't go ultra-cheap like in the thread since those who used Rustoleum had to go up to 10 passes of rolling paint then sanding, since must lay down Rustoleum in very thin coats.
I just painted repaired dents on my 1984 M-B. It was 2 door bottoms (below horizontal trim) and the top rear of quarter panel and trunk on one side. Matching the color on the later was tricky since didn't want to repaint the whole trunk or quarter. I used Brightsides white, after their Pre-kote primer. First pass was too-white to match, plus some runs I sanded out. I had some leftover gray 2-part, but also a polyurethane, so used a few drops of grey until test-dabs appeared to match the car's white (or off-white). The second spraying effort came out better with no significant runs. That was with a touchup siphon gun which flows about half the volume of a normal gun. It seems the trick is to initially lay it down in a thin coat, with just one pass of the gun. Wait at least 30 min for the 1-part to air-dry. I did two coats this way. I waited 45 min before the 3rd coat, which I laid slightly thicker (one full pass horizontal, then quick vertical pass - don't forget to rotate the fan spray). The existing paint then seems to "suck up" the solvent before it runs. That was my prior experience with rattle-cans on oil pans and valve covers, and the instructions on those cans. If a part is off-car, like a fender, you can sometimes re-orient it while wet to make a run horizontal, then blow it flatter w/ air and even spray a little more paint to level it. Has also worked for me with rattle-can lacquer paint that flows out well, indeed so well that it loves to run if you get too thick. My painted cars aren't perfect, but not $20K either, and I see much worse in parking lots of 10 year old cars with failed clearcoat (strong sun here), especially dark colors.
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