This from 2006 "Code Rousseau" French Highway Code
Non-presentation of documents fine 11 - 38Euros
Excess of 1 - 19 kph over limit 68 euros on the spot or between 135 and 750 euros if you dispute it and lose. 1point on licence
20 - 29 kph over limit 135 - 750 euros, 2 points
30 - 39 kph 135 - 750 euros, 3 points, or possibility of up to 3 years suspension of licence,
40-49 kph 135 - 1500 euros, 4 points or suspension of licence for up to 4 years
50kph plus Automatic court appearance, 6 points, 3750 euros fine.
That having been said the only time we've been done, we were actually doing 30 kph over the limit but he wrote it down as 18 and just gave us the on the spot fine of 68 euros and no points. (English car, french reg, UK licence). As in the UK - be polite, do a sort of shrug equivelent of "It's a fair cop" and generally they're OK.