The cheapest SEO is when you do it yourself. SEO is an ongoing process and to hire someone to do it means that you must pay them for the life-time of your site.
Take a half an hour out of your schedule, read beginning SEO and an SEO Checklist and start right now. Make a goal to become an expert. An expert SEO webmaster is one who has more than one high quality site to be used for backlinks to your other sites.
If you do pay an SEO company, what you'll be paying for upfront is high quality backlinks, error free html, keywords and the other webmaster's time. Your site needs to be updated and there needs to be a gradual increase in high quality backlinks throughout the life of your website in order to keep your high rankings at SEs.
When you start doing SEO yourself, one must get to the point where the only things you need are updates and high quality backlinks. Become an expert on obtaining free high quality backlinks now, it's easy with a little advice found at SEO sites. For example, backlinks at free blogs work well.