I'm an economist (aspiring economist, if we are being technical) and my curiousity has been recently piqued by search engine optimization services.

I had the following questions.

1) What industries most widely use search engine optimization services?

2) Is it small business or big firms that use SEO services typically?

3) What is SEO pricing typically dependent upon and how much is it on average?

4) Do SEO services really increase traffic? Is there a dataset I can get a hold of for this sort of thing?

I am not looking for answers to the questions necessarily. Although answers would help, a nudge in the right direction is more than sufficient, and greatly appreciated.

Thank you!