Although interior house paints can be purchased in pint-size samples or five-gallon buckets, the standard measurement is by the gallon can. A gallon can of basic water-based paint can be found on store shelves for less than $25, but the additional cost of a primer coat should be considered. One-coat paint/primer blends for most DIY projects should cost between $30 and $60 a gallon, including customized tinting. VOC levels should be lower as well. For high-end painting projects featuring designer colors or full-gloss finishes, expect to pay $60 or more per gallon.
A gallon of paint costs $15 to $40 and covers about 350 square feet with one coat. Designer varieties can run over $100 per gallon. Your contractor pays 50 percent less than what it runs in a home improvement store. You’ll also need to spend another $20 to $100 on supplies like brushes, rollers, masking paper and plastic. A professional will have these items on-hand.
Paint acts as a protectant on wooden siding, slowing down the rotting process. This makes it incredibly important to fully coat the undersides of corner boards on the exterior of your home. Whether you’re undertaking a DIY house painting endeavor or have hired professionals, you should check corner boards and shutters to make sure they’re fully painted.
Before hiring a painter, you should get a detailed estimate from them. This should include costs for labor, materials, paint, and the timeframe of the project. Covering details upfront will prevent issues from emerging later on. Ask the painter if they charge for travel time, whether their estimate includes pressure washing and caulking, and any other questions you might have.
However, some water-based paints are easier to work with than others. The first coat of paint may not provide complete wall coverage, or the color may not be quite as saturated. Some of these problems can be addressed with a separate primer coat, but this is a time-consuming project. The ideal solution is using a higher-quality paint that combines a primer and a finish paint in a “one coat” product. 
Another type of paint is on the market that claims to be a primer and paint in one. Basically, it’s just thicker paint and costs twice as much as regular paint. Most professional painters aren’t fans, but if you have exterior walls in excellent condition already, and the self-priming paint is the same color as the original paint color, it could certainly save painting time with one coat going on instead of two.

If you are painting for a client, skip the paint in the bucket, and use large trays. For faster completion and still quality results the 5 gallon bucket and strainer work great as well as a wood filler, 5-in-1 tool, screw gun (drill), metal joint compound tray, metal putty knife, wide drywall knife, mini hand-held paint containers (for brushes), and a mini-roller and corresponding nap (for edging, after cutting in with brush on the bottom, top and sides of wall, and the sides of ceilings, so that you don't see brush lines).
A single gallon of paint can run anywhere from $20 to $80 per gallon. Professionals get a contractor discount of anywhere from 25 to 50 percent. For rough siding, like stucco, you’ll need to use 10% to 20% more paint. Some textures, like wood and stucco, require extra paint because they have more surface to cover in a tiny area compared to smooth siding. Use the highest quality exterior paint your budget will allow for your project. It will look the nicest and save you money by offering better coverage and durability. Better coverage means fewer coats and fewer work hours. You’ll also go a few years longer without needing to shell out more cash for a new coat. How do you know how much paint you're paying for? Here's some helpful math:
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