It depends on your budget also, some people with big pocket don't mind spending  since they want results quickly, some people goes to freelancer for cheaper service, and some who have time try their own hand in Seo.

In the end, it all depends on your pocket.

I've heard that those prices were low, I wonder if i'm missing something. Which freelance guys offer SEO for less than 75?

Totally agree with Karmadir , but this is even worst in SEM because there are clients who dont want to spend $ 100 dollars in a PPC campaign because they think that is very expensive.

In SEO I optimize sites that nowadays has 3.5 millions visitors per month and when they read about that accomplish they want the same for its sites , however every nich is dofferent of course and very hard to explain why I cannot send you 3.5 millons to your site that sells cars only paying $ 100 on SEO .

Sometimes clients has no right and you have to explain them .