According to Glassdor the average salary is about €39k[1]. In my experience a "good" salary for a french engineer is between €42-50k. Housing isn't cheap in Paris - count about 12k-18k for 430-650 sqft. Going out isn't cheap either.
I've lived a year in SF and had definitely a better [salary]/[cost of life]. It was 5 years ago though.
That's meaningless since costs vary along different axes. I just mean objectively software engineers have not traditionally been valued the same in Europe as they have been in the US.
