"Under the current system, France offers employed mothers 16 weeks’ paid congé maternité (three to six before the expected date of birth and 10 to 13 after). The total allowance increases to 34 weeks for twins and 46 for triplets.
Women receive 26 weeks, instead of 16, from their third child. They can choose to return to work sooner, but must take a minimum of eight weeks, including six after the birth, to receive pay.
The leave is paid at 100 per cent of salary, capped at €540 net per week. This is the equivalent of a €2,885 gross monthly salary."
2855 Euro gross is fairly normal in France for the bulk of the people who would need this assistance. The $650 is net take home, not gross. You also have to take into account that in France lots of things that are very costly in the USA are free and so you can't directly compare the compensation for paid maternity leave with the income of the median woman in America, that's apples and oranges. You should compare it with either the median pay in France or with the average paid leave compensation in the USA.
Finally, there is the duration of the leave to take into account 16 weeks is going on three months, which compared to the 6 weeks (or less) of maternity leave in the United States is pretty generous.
Is it costly for mothers? Well, that depends on the income of the mother before the maternity leave. But France has figure out - rightly - that if you earn lots of money that you need this assistance likely a lot less than if you do not. And so there is a cap.