No kidding .... I have literally thousands of dollars worth of paint in my inventory and still always need more with each new project !

Mark.
I used to paint brass steam (as well as re-quarter and adjust valve gear) for a few years until I just got burned out on it. I did N&W, PRR, Southern, and Western Maryland and a little C&O. I added the marker jewels, slide glass windows, crew, and small details like oil cans, cold weather curtains, very small lights mounted to the back of MV Lenses in the headlights and real coal.
For me it was the research into what colors the engines needed to be weathered with. The variations in coal, water, and sand made a big difference.
I found that my big limitation was decals. One client had me painting 10-15 pieces a month for PRR, and another wanted 3-5 a month in PRR. After the first guys engines were repainted, he had me doing his large passenger fleet. I cleaned out all the hobby shops from just below Baltimore MD down to Midlothian VA. I finally got one hobby shop to get me an account with Microscale and Champ so I could order 20-30 at a time on credit.
Mark & Y3b
Been there, done that, have the T-shirt and even saw the movie...twice!
While living & painting in Mobile, Al I averaged 10 locos of all kinds a month. I could paint the loco and weather it if desired, and sometimes use a total of 22-25 colors and tints to do so. Generally if the client bought the decals, detail parts, and provided some pictures he got a good discount, if not, it was the price of all that, plus the paint and weathering job itself. On some locos I could make several hundred. I've had quite a few locos pictured in magazines, and even had them featured in one article. I had even been asked by Overland's painter at the time, Gwynne Burch, (who is still a very good friend,) to paint for Overland. I turned it down.
When we moved to Birmingham, Al, I had to give it up for a while, as apartment living wasn't condusive to mass production painting. When I got back into a house I started up again, but after a while, 10 years, I gave it up. It wasn't that I was burned out, as I set the schedule so I wasn't painting the same scheme multiple times, it was that it had gotten to the point that I had NO time for MY hobby. Since then, I have watched over $5000.00 worth of paint "die" over the years. I'm slowly buying paint as I need it for the few jobs I still take every now and then.
One good thing is that I have built up a good decal collection over those years. I have several large plastic sealable loaf of bread sized boxes loaded with solid sheets and another one with partial sheets. I'm fixed in the decal dept. Many I got through clients, LHS's and some from E-bay. In the 2 years before Champ, closed I think that I got over 200 sets of decals from them, mostly cars for SRR. All the rest of my decals are for other SE roads, a few western, and eastern roads round out the bunch.