I had about $1000 into just paint for my stepside truck. That was kinda cheap paint too from TCP. I used urethane base coat/clear coat.
There are so many little things. Sandpaper, blocking tools, spray guns, mask filters, tape, seam sealer, various types of primers, body filler, etc.......... I bet I had another $1500 into that stuff. In a shop the cost of tools would wash out pretty quickly but some is just unavoidable. I bet I spent well over $100 on solvents to clean up spray guns alone. $50 rolls of sandpaper and $12 cans of etch primer add up quick. Then your time.......
I know a really top quality paint guy and watched while he did a black paint job on a mid 50's Chevy pickup. The end result was just incredible, there was not a spec of dust or a single visible sandpaper scratch anywhere and it was like looking at a mirror. He said that one cost 20k and that was 15 years ago.