Far and away, the least expensive traffic you'll ever have to your site will come through the "natural" or "organic" search results on Google or Yahoo.
To get traffic to your site for the search words or keywords that are important to your organization, you need to do what is called Search Engine Optimization or SEO.Fortunately, this is something that the average small business owner or staff person can do himself or herself.
Yes, it takes time and knowledge.
But it doesn't have to take much money that is, unless you are trying to rank high for very competitive keywords. SEO has two parts, each equally important:
1 webpage optimization and
2 getting links back to your website