Buy It With Your Eyes Open
Seems like everybody is selling SEO. You wake up one morning, and your dog has a new gig selling SEO door-to-door. He's partners with a pony. You're already getting robo e-mails from SEO companies. Soon they'll be robo-calling you 24/7. I haven't yet seen my first SEO billboard. It's coming.
But really, do you need it? Not necessarily. If I were you, I'd look at this just like at any decision to put money back into the business.