It depends, I work for an seo firm so I don't personally. But mostly I recommend it for people who don't know much about marketing or don't have the skills or resources to learn it on their own.

Even if you can learn seo, people go to seo firms because they want our expertise. We can deliver results whereas if you do it on your own you can only hope to get some sort of success partially on luck.

It all depends on your budget, there's seo work to be done for any amount over $10.00 - from something as simple as a basic link to more encompassing seo work involving viral link building campaigns, reprogramming and design of your site, ppc campaigns etc.