SEO....how much does it cost?

[softball]

I am up to my neck in launching a new website. I am buying magazine advertising that I know works like a hot damn. I have been considering some SEO, but have no idea how it gets billed. In the past, I have bought traffic for actual money and got nothing. I am not really in the mood for another experience like that. So how do you know you are getting the real deal....anyone can deliver clicks, but if my sites are "optimized", how do I know its worth it. I want some seo guys to chime in and explain your business and how it works for those of us who are just stumbling along blindly but doing well.