I do not mean to come off with an edge, however, this is an article for people hiring low bid, uninsured, one man band painters out to "hustle" a buck. Hire reputable, established companies or hire someone you know or have a strong referral from. If you are arguing over watered down paint, ticky, tacky deposits, buying your own supplies, you have hired the wrong guy.
When coverage is difficult to estimate, add more rather than less when doing DIY wall painting. You can always pour the leftover back into cans. For large jobs, use the bucket and a roller screen rather than a roller tray. It’s much faster to load your roller with the screen than to use a roller pan. Simply dunk the roller into the paint bucket, then roll it along the screen until it stops dripping.
Second coats on similar colors are almost never recogicnized as being needed until the coat is applied and has dried. ONLY THEN WILL YOU SEE WHETHER IT NEEDS A SECOND COAT or not. Yes, painters can use a cheaper paint then what you paid for. That is solved by getting your own which, I would charge extra for because I will always have to go get more, or add second coat because home owner tried to skimp on paint, or they got the wrong color etc...

I used to work at Sherwin Williams. The best thing to do is specify that you will purchase your own paint. If the painter objects strenuously, he was plaanning on making money in the ways this article outlines. You might pay a few dollars more for paint, but you will have control of the quality of what goes on your walls. Never skimp on paint quality.

When painting the trim, you don’t have to be neat. Just concentrate on getting a smooth finish on the wood. Don’t worry if the trim paint gets onto the walls. You’ll cover it later when painting the walls. Once the trim is completely painted and dry (at least 24 hours), tape it off (using an “easy release” painter’s tape), then paint the ceiling, then the walls.
Paint gets more expensive as you go from flat to gloss. The difference is usually around $1 per gallon per sheen upgrade. Flat is the cheapest, then matte flat or eggshell, satin, semigloss then gloss. Paint also gets much more expensive the darker it gets. A white or neutral paint color can be as much as $20 less expensive than a deep base red or blue.

The speed with which a painter can complete the job will determine its final cost, but time is difficult to estimate. Some painters have more experience and cover a wall faster, but some are more methodical and take more time. Most painters should be able to cover about 100 to 120 square feet of flat surface in an hour, unless they are working on a very large wall. Wood or plaster might reduce that amount to 80 to 100 square feet. You should also consider the time required for a first coat to dry before a second coat may be applied. This will add time as well -- anywhere from one to 48 hours depending on the paint.

House Painter Company