I've seen this argument over and over again, as if "more expensive to employ" directly causes "lower salaries". I don't think the two are directly tied. If it costs you $150k to pay $90k employee salary, for example, that doesn't explain why there are $300k+ employees in the Valley (while that amount of money is kind of preposterous in Europe, though I'm sure some have it as well).
In my mind, it's rather a combination of:
1) European employees not willing to work much more than 8h a day (whereas US folks tend to work themselves to the bone)
2) European employees living in cheaper countries (so the HR has a ready-made BS answer to "why do I earn less than my US counterparts)
3) What I believe is a bit of disrespect towards non-US employees. If the company's based in the US, the upper management's going to be US, and I often feel like the attitude is somewhat "your education must have been worse than the US one, so you'll probably be worse than US engineers" kind of a deal. This is, of course, very subtle; I've personally never felt that from my immediate co-workers, but the salary policies seem to be geared this way.
Of course, being more expensive to employ doesn't help. I'm sure it's one of the contributing factors too. But, as a European, I feel like it's far from being in the top 5 reasons for the salary discrepancies.