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.

With that said, here's the reality of that particular scenario. Painters do put water in the paint, but not for reasons you would think. Some materials need to have their viscosity manipulated in order to slow drying time, allowing gravity to 'smooth' out the product for a better finish. It also prevents 'drags' and 'sausages'. I personally try not to do it too often, but from time to time I have to. I want my client to have a proper finish.
In some cases, professional painters may include additional charges for specialized equipment that homeowners can't purchase on their own. Because professionals have licenses and access to such equipment, it's simpler to let them get those themselves. But providing some of the smaller equipment and extras directly really can help to cut down on the total cost of your project.
Research the licensing requirements for your area. These laws governing professional contractor work will be different depending on where you live. It’s important to be aware of these laws and what they mean from the outset. Contact the licensing board for your state or district to find out more about what you’ll need to do to get your business off the ground.
Recently I had the outside of my home painted. The contractor wrote a good contract, but I failed to realize that some things were not in it. It reminds me of the car dealer who offered a good price on a new car but failed to mention that it did not include tires. My contractor failed to specify that lattice under a porch was included. So the painters did not paint it. To his credit, he did instruct them to paint it when I brought it to his attention. If I had the job to do over again I would look for an individual who came with referrals from happy customers rather than a franchise owner.. 

House Painter Co