I get the majority of my customers through organic SEO. The marginal cost of a customer from organic SEO is zero. The amortized cost is almost impossible to calculate -- I SEO well because I spent years learning the technical/marketing aspects and, e.g., blogging (which has a side effect of getting me domain trust), writing useful resources for my niche, coming up with a content creation strategy, etc. (The great thing about content creation is that if you're making evergreen content you pay for it once and then it keeps getting you customers/links/etc for forever.)
AdWords, on the other hand, is pretty easy to calculate marginal CPAs for. My previous experience with Google leads me to believe they understate my conversions with them, but just reading the number off the AdWords report, I spend about $12.25 per customer. Since I make about $29 from each customer after all costs but advertising, I'm quite willing to do this all day long.
For those of you who are more interested in cost per signup rather than cost per paying customer: 22~24 cents.
SEO is a deep, deep field though, so this question is close to "Do you know any resources about programming?"
I spend a lot of time talking about my particular take on it on my blog, although I don't have a good entry point for you there... Hmm, I guess this post from 2007 is the first big one about it.
Things have changed since then -- the content site that I developed for that project was merged into my main site, its gotten a lot bigger, the ROI is no longer prospective, and the overwhelming majority of the manual labor is automated or outsourced.
The best two free SEO resources I know of are SEOmoz.org and seobook.com. They probably taught me most of what I know on the subject. (Both also have paid portions. I pay for SEOMoz, which is mostly tools. I used to pay for SEOBook, which is mostly for forum access to people who know what they're doing. I no longer pay for that because they made me a mod, greatly bringing down the average quality of the moderating staff.)
Start with the easy stuff like a) creating keyword-laden browser titles for all your pages, b) having good text content in your pages (especially stuff like your "about" page(s), c) linking to your site from any other web properties you own (personal blog, etc.), d) make sure the links have good anchor text, and e) blogging.
When you blog, take time to write good content–and make sure your post titles have keywords in them (keywords in header tags help).
You might consider an SEOmoz subscription just for a month or two. Go through all their articles and you'll learn a lot.
I get the majority of my customers through organic SEO. The marginal cost of a customer from organic SEO is zero. The amortized cost is almost impossible to calculate -- I SEO well because I spent years learning the technical/marketing aspects and, e.g., blogging (which has a side effect of getting me domain trust), writing useful resources for my niche, coming up with a content creation strategy, etc. (The great thing about content creation is that if you're making evergreen content you pay for it once and then it keeps getting you customers/links/etc for forever.)
AdWords, on the other hand, is pretty easy to calculate marginal CPAs for. My previous experience with Google leads me to believe they understate my conversions with them, but just reading the number off the AdWords report, I spend about $12.25 per customer. Since I make about $29 from each customer after all costs but advertising, I'm quite willing to do this all day long.
For those of you who are more interested in cost per signup rather than cost per paying customer: 22~24 cents.