Dickens tells us that "Darkness is cheap, and Scrooge liked it."
The "and" is significant. Dickens doesn't say "Darkness is cheap, so Scrooge liked it." There are, instead, two parts to Scrooge's thinking about darkness. First, and most obviously, it saves him money, which refers to the first part of the statement. The narrator tells us that the entry to his dark house could easily use six gas lamps to light it, and miserly Scrooge would prefer not to spend money on such frivolities.
But it is not simply that darkness is cheap; Scrooge actively likes it. The implication is that even if he were given free lamps and gas, he would choose to sit in darkness.
This embrace of darkness symbolizes Scrooge's choice to live cut off from other people and his own feelings. He finds it easier to live in the darkness of not seeing the suffering and joy going on all around him. His emotions arouse painful feelings, so he prefers to isolate himself from anything that might, metaphorically, light them on fire.