I usually tell that I can make little changes in the image, like removing wires or electrical sockets, things like that. But when they want to have objects removed that are in front of something that is not a smooth gradient or simple pattern, that will be extra. And for complicated backgrounds, that will be outside of my ability, but no problem, because I can supply the name of a digital artist that can do the job. Of course a good artist will charge $200+ an hour. That price usually gets them to make sure everything is the way they want it on location.
PS, I do remove exit signs whenever I can, and that is what I spend the most time removing when doing that kind of work. Here I dont charge because I would remove the sign anyway for my own use.