Deciding which paint to use has gotten much easier now that acrylic latexes have pushed oil-based paints almost to extinction. The acrylics offer superior performance (they don't harden with age, the way oils do, so they move and breathe without blistering), they don't mildew as readily, and they emit fewer VOCs, so they comply with new air-quality regulations. They also work over both oil- and water-based primers.
Unless you like textured walls, don't paint over dust. Wash the walls with trisodium phosphate (TSP) or a mild cleaner, like Jasco's TSP No-Rinse Substitute. For smoke-stained walls, Toto uses a 50/50 solution of bleach and water. To clean a wall, use two buckets. Load up your rag or sponge from your cleaning-solution bucket, and scrub. Before redipping, rinse the sponge in a second bucket filled with clean water.
The BBB isn't a painter-specific group but rather an organization that is dedicated to ensuring that consumers can access information about anyone they do business with. The BBB also rates companies based on a Standards of Trust scale, which goes from A+ to F. A company doesn't have to be a member to get a grade, and it's a good idea to check to see how your contractor is scored as well as its membership status.
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.
Buying a can of premium paint, then bait and switch over to low-line products? Again, complete nonsense. Think about it for a moment; the Painter needs 5 gallons of wall finish. So, he buys one can of premium and the rest 'cheap'? How is he going to hide the 4 other gallons? What's he going to hide it in??? He only has ONE gallon of premium. It's not as though he's going to keep older can labels, they would be covered in paint of a different colour.