Today I checked usage from the first 2 weeks: $0.20
I originally estimated a $0.08/day (based on ~10 commits per day at ~4000 tokens per commit), but it's lower because diff's are often tiny (a few dozen characters), and I only use it for a third of my commits - the rest require thoughtful (human) messages.
In any case, this was a fun experiment, and a cheap way to save, I dunno, 5 minutes a week :)
Usage/cost graph: https://imgur.com/a/X7Ldqq1
Initial cost estimate: https://news.ycombinator.com/item?id=35303100
Source code: https://github.com/stevecondylios/gpt-generated-commit-messages#setup