Since the AMS website has put on its frontpage a study of faculty salaries at US PhD granting math departments, here are a few simplistic comparisons.
For the group of “Large Public Universities”, the New-Hired Assistant Professors get at least $70,000, with a median of $84,700. In that same group, the median for Full Professors is $126,900.
For the group of “Medium Public Universities”, the New-Hired Assistant Professors get at least $50,000, with a median of $76,200. And the median for Full Professors is $106,200.
Finally, for “Small Universities”, the New-Hired Assistant Professors get at least $50,000, with a median of $68,500. And the median for Full Professors is $93,500.
At Private Universities things are much higher still.
I haven’t searched for elaborate statistics. A position of Lecturer at Leeds mentions a minimum salary of about $56,360, while a position for Chair Of Mathematics at University College in London is mentioning a minimum salary of “£65,179 per annum inclusive of London Allowance”, that’s about $109,455.
So it’s a bit less than in the US. Obviously, some figures comparing the cost of living on both sides of the pond should be thrown in to put things in perspective, which I won’t do myself for lack of time… If this numbeo website comparison is to be believed, the cost of living is much less in the US than in the UK, so academics in america are really much better off than their british counterparts. (One thing not mentioned by numbeo are health insurance costs, which I believe are much higher in the US, perhaps to the point of evening things out.)
A tenured Assistant Professor there is roughly what’s called a “Maître de Conférence”. Since they work in public universities, their salary follows a publicly available grid (the same at large and small universities) which starts at €2,102.15 gross monthly, so that’s about $34,862 annually (at the current €/$ rate). Impressively low compared to the US figures. And after ten years the grid reaches €3,116.19 gross monthly, which is about $51,678 annually. That’s for the salary part, some of them also have an extra $2,000 a year for scientific merit, I think.
As for “Professeur des Universités”, which roughly corresponds to Full Professor, the corresponding grid starts at about $50,525 gross annually. It’s difficult to say after ten years where it gets because of all the different cases. Something around $82,000 probably, noticeably below the UK and the US. The very top of the grid reaches $101,360 (obviously for very few people, a couple of Nobel Prize winners maybe).
Since France and the UK have very roughly the same cost of living according to that numbeo website, the french researchers are those with the smaller earnings relative to cost of living (by some distance).