2.5x seems really high, but 2x is usual for software engineer salaries and up.
For example, an average software engineer can receive a gross salary of 50 k€ which translate to 34 k€ after social contributions and income taxes (in a worst case scenario) and a total cost of 71 k€ for the company [0].
Using this simulator, I can reach 2.5x by putting in a gross salary of 250 k€. Of course it's mostly because of the high income tax in this case.
There are also strong incentives for startups, most notably reduced social contributions during their first 8 years [1] and tax credit for a third of research expenses [2].
Admittedly, this is probably more complex than needed but I think it is still way less expensive than hiring in the US.