I got an estimate done for the roof on my house a few years ago. Got three estimates and to my surprise they were all within 10%. I have been estimating paint jobs for over 25 years now and the difference between quotes is always much greater.
Why is this?
Estimate a roof or flooring is much easier to estimate than painting, it is mostly based on square footage. Painting however has many more variables, there is the square footage of the walls, linear footage of baseboards, number of window, doors, railings, etc. This make it more complicated in doing a proper painting quote. Most painters do not do a proper painting estimate, they either use square footage of floor space or a guess estimate. This means the estimate is either too high or too low. If it is to high he probably will not get the job. If the price is too low he will either loose money or find a way to compensate this lower price by not doing a proper paint job.
It is important for the customer to make sure a proper estimate was done. You can usually see this by detailed the quote is. If there is just a lump price chances are it was not a proper estimate. This could result in problems down the line, the painter will either cheat or ask for more money after the job has started.