is Mort a millionaire?

Multi-billionaire.

Probably well off by any normal standard, but he's going to lose big money in the divorce and any writer has an uncertain and uneven income.

So he's probably got more money than you or me, but probably worries about money as much as anyone.

It’s based on a Stephen King story so I assumed the prolific writer of many novels and short stories in the movie was certainly well off too.