How Much Should Commercial SEO Cost?

I've been referred to a service that charges approximately $5000 for website design and SEO. I've checked out the websites they have made for others and they seem to get results with high search rankings for their clients. Is this a competivie price though? I can design my own website fairly easily, I really am just looking for SEO. Is there cheaper, but effective ways of doing this (other than doing it myself)?