A recent post by Rand over at seomoz looked at the marketing spend between SEO and PPC with search engine marketing campaigns.  

In this blog Rand was discussing that whilst the US spent approximately $10 Billion on PPC marketing, whereas SEO accounted for $1.3. Overall, SEO accounted for about 11% of internet marketing spend, with PPC at a whopping 87%.

Later in the same article it was remarked that SEO drives over 3/4 of web traffic, which makes the disparity in spend even more confusing.

I think that there are a number of elements to this to be considered though, as it's not as cut and dried as that.

I think that PPC will often target phrases and searchers that are in a buying phase, or looking for a product - so focus far more heavily on phrases that can potentially make money. Whilst this isn't always the case, as there are bound to be PPC ads on some crazy phrases that have very little chance of resulting in a sale, PPC activity is very results and conversion driven in all areas when its run properly and as such focuses on these areas that they can make money.

SEO is somewhat different. With SEO, you work to build up a sites authority in a given niche. Whilst you often optimise to appear prominently for particular phrases, and will often focus on areas that will make a client money, there is often a knock on effect to help the site rank well for phrases that are not so monetised.

You also don't just focus optimisation on areas of the site that can be monetised (or we don't). Some of the best results on competitive phrases come from informational resources that really do provide value to a searcher, but are more difficult to quantify a return on an investment. In situations like this, it would likely be a waste of money to bid on such a term, but people that find information on your site useful are likely to come back again in the future, and may end up converting and using a service or buying a product based on value you had previously supplied to them in an information seeking search.

One of our clients recently worked out an approximate cost per visitor based upon our monthly fee and the number of additional visitors they have gained as a result of this, and found that it was about half the price of his average cost per click that he pays for on his PPC campaign.

SEO is definitely improving its traceability and looking to improve the methods used to quantify ROI, but PPC is at the moment much easier to actually measure definitive returns on investments made. However, SEO has a lot more traffic driving potential and should ultimately result in more conversions for your website, so the large disparity

In short, whilst it's not always as simple to quantify, SEO can generate more visitors for less cost to your website than PPC - it's just more difficult to measure the specific ROI from investments made in SEO.