How much should SEO cost?

How much do you think SEO should cost? And why do you see it as a cost? It's a highly targeted investment if done professionally and correctly.

Figures (and services) for SEO vary wildly. Someone starting out may be selling cheap SEO for less than $500, an agency may well expect figures to be in the 6 figure region before they get interested.

I'm mid range these days based on my experience, knowledge and results. I'm happy here.
So what price do you think you would pay for good SEO?