Yes the pricing does change, and quite significantly. I have worked inside the paint industry on counter sales outside sales and application for over 10 years. One thing many people do not understand is, the resins in the paint are more expensive to produce the higher the sheen goes. Therefore the company is at of higher cost making the paint, which in terms they charge more for the paint. I have seen a 15$ variance between flat-semi-gloss it is not uncommon and is not unrealistic to pay more for a higher sheen. Another thing to add is when doing samples on your wall prior to a painter coming is a good idea, however do not do them in huge squares with heavy coats, remember you are just wanting an idea. A lot of times the issue comes up of the paint not covering the sample coats, in fact it is covering quite well, however the paint sample applied is often times much darker than the wall color, creating a contrasting difference from the lighter surrounding wall and the sample placed on the wall. I recommend getting a piece of sheet rock and using it for the samples so you can move around with it etc.
I managed commercial construction projects for many years, have built and remodeled several properties, and never once have I encountered any of these scams. The tone of this article is deeply troubling. The author seems to be saying that ALL painting contractors are inherently dishonest, and that has not been my experience. The underlying advice here is sound: get it all in writing and cover as many contingencies as possible--so pointing out potential pitfalls like coat coverage is helpful. But do that in the spirit of clear communication of expectations, not with the expectation that the person you are hiring will try to cheat you at every turn. Not every contractor takes outrageous advantage of change orders; not every contractor will sneak past necessary preparation and/or repairs. Contractors of all sorts get a bad rap as it is; reinforcing a stereotype with articles written from this point of view just seems unproductive.
