Guy, there are two aspects of SEO to consider. One could save you a little more than the other, however neither should cost the earth.
In terms of on site optimisation, many factors can be edited by the webmaster (in small businesses, typically the owner). A lot of suggestions can be found through the PRO campaign management tool, however there are plenty of suggestions on the Beginners guide to SEO http://www.seomoz.org/beginners-guide-to-seo
The main factors you should focus on, for on-site optimisation are;
- title tags
- h1 tags
- keyword usage on site
- keyword match in url
- image alt text
- internal linking keywords
In terms of off site optimisation, there should be a small budget set aside. This should not necessarily be spent on paid links, as these will over time be picked up by Google, and your websites performance will drop. However, focusing on generating good, interactive and/or graphical content (for example, see ours: http://www.evoenergy.co.uk/uk-energy-guide/) is the way forward, as it attracts a lot of links from design-related blogs and galleries.
Perhaps you could let us know what niche you are in, and we could all suggest a few ideas for you?