The salary of a SEO depends on the kind of company they are working in: an agency or at the advertiser, among other things. It also depends on SEO skills, if they only works on SEO, if they also do SEA, if they work as a consultant or as an integral part of the workforce…
The latest French studies show that junior SEOs earn 25k/year, confirmed SEOs earn around 35k/year and expert SEOs earn around 50k/year.
Obviously these salary scales are not authoritative and are given for information only.