Its all depend on your experience. If you are the expert then you may get more then 200$ per website for a month. Meanwhile you can get other project with same rate. So in SEO there is no fix minimum or maximum.
SEO project manager, SEO expert are the more sersached terms in job portals. HOwever, if one has sound undersatnding of SEO and new changes in algorithms, he can earn better by working as freelance SEO expert.
SEO role & salary depends upon the years of experience in relevant field. Coming to Future of SEO, SEO will have very bright future. No need to worry about the future of SEO. Bother about the relevant experience.
Today search engine optimization are gettig very popular in many countries. Even governement sectors are hiring SEO services. Today every businessmen is much aware regarding excess number of players. It means maximum competition in every product or service.
About different post in seo:- The best post is content writer. If your english is very strong and without any grammatical error, then you can earn good money while sitting form your house or in any company. There is a reason behind this, that is there are three pillars in seo. they are original content , backlinks and social media. so you can imagine how much money a person can earn in seo. the other post is head of seo or senior executive seo analyst. He earns very handsome salary. But for this position the basic concepts shuld be very strong. He should be capable to talk to foreign clients. He should satisfy all the foreign clients.
If you don't know anything about SEO then you will be hired as a trainee then you will be promoted on the basis of your performance.
SEO is rapidly growing field you will be holding senior position after 2 years.