Origin/meaning of phrase “a martini for the rent”

I just finished reading Cornell Woolrich’s Rendezvous in Black (1948) and 2x in the book he has a character say he’s having “a martini for the rent”. I didn’t note the page numbers unfortunately so I can’t produce the exact context but I believe they were drinks ordered in a bar. I’m not finding any other uses of the phrase and I can’t guess at the meaning…except maybe because gin was once a very cheap, lower-class alcohol (though certainly not but the time of publication of this book)? Maybe it was a cute way of ordering a dry martini because vermouth is the pricier ingredient? Anyone have any ideas?