For older homes the wall panels and ceiling lines can take a ton of caulk to fill and prep so that the result looks good. Then count on a couple coats of Kilz and then your final colors. Lucky to get out the door for $1.50 per foot in this case. Newer homes closer to $1.15 as there is less “cheap” looking paneling to prep and hide with caulk.
There’s always something that gets forgotten. Last dude painted all my interior doors and didn’t take the knobs off. That was fun.