Many factors will effect how much a person earns in a particulr job, experience, whether they are employed/self employed, how large the company is they may own or work for etc I would add that at the moment I would say the demand for web developers is higher especially in certain languages... Hope this helps??
There really is no specific rule! But, it depends upon how big the project is ..and your customer's budget
It totally depends on the customers need what he actually wants from you and also on the size of the project size.Web developers are earning good money in small projects too and vice versa .
I've seen both earn a lot. Skilled web developer are more sought after however, because seem harder to find.
It also depends on the complexity of a project. You cannot compare a graphics artist who is asked to design a logo, with a web-developer who generates a site from A-Z with complex code with multiple pages. The reverse also applies. A graphics designer may have a larger project who designs several images across a website/poster/event, and the web-developer may have a simple task on a particular page. This doesn't apply to these two careers. Any job has its eases and difficulties.
I think It depends on many factors, such as their skills, Jobs, experience, complexity of a project, how large of the company
Apparently you are reading challenged. SEO is NOT a person, NOT a graphic artist, and NOT a web developer. So that makes YOUR post SPAM.