A gallon of decent paint costs less than dinner out, which is why painting is the first thing most homeowners try to do themselves. Sometimes that's the right call. A single bedroom with sound walls, no trim work, and a color close to what's already there is a reasonable weekend project.
The cost shows up in the rooms that aren't like that. Walls with nail pops, old patches, and glossy paint need sanding and priming before the new color will stick. Ceilings need a different roller and a different product. Trim needs a steady hand and a brush that costs more than the paint. Skip any of it and the job looks fine for a month and then starts to show every shortcut.
Then there's time. A professional crew repaints an average living room, including prep, in a day. The same room takes most homeowners a full weekend plus a couple of evenings for the second coat and touch-ups, and the furniture lives in the hallway the whole time.
Our rule of thumb: if the walls are in good shape and you're only changing color, do it yourself and enjoy it. If there's repair work, trim, ceilings, high walls, or more than two rooms, get an estimate. It's free, and the number is usually closer to the DIY cost than people expect once the materials, tools, and weekend are added up.