Fast, Cheap or Good - Pick Two

Programmers use that phrase when describing peoples software development options, implying that you can't have all three.

For SEO the phrase: "Thorough, Effective or Cheap" seems to be appropriate.

 Why do I say that?

Cheap
This is easy, SEO is not cheap, but it should not be outrageously priced either. If done correctly SEO yields a good return on investment.

Thorough
Refers to long tail search. Invest time to identify keywords for your market. Remember that people searching for our product and service might not be "experts". Therefor the terms they use won't always be the ones that you think. Keyword research tools are so important to SEO. Thorough SEO finds words and phrases that you don't know about.

Effective
Just randomly pursuing every keyword / phrase does not make sense. Some words will lead to better conversion rates than others. The trick is figuring out which ones to spend more effort on. Some words will bring eyeballs others dollars. Effective SEO goes after the latter.

While you can't have all three "Thorough, Effective or Cheap" you can have "Thorough, Effective and Reasonable" SEO.