I work with an ecommerce company that was going years before I came on the scene so can't give their numbers but I can pass on some advice handed on to me - invest time in doing your own SEO/M and getting in the organic SERPS from the start. Thats the long play that will serve you well over time. PPC and other forms of online marketing could give you an initial boost in the right direction but get your SEO right from the start and it will pay for itself many times over.
I absolutely agree with morehawes. It's kinda like the movie "field of dreams", if you build it they will come. Well you build it and they will come. But the important part is how you build it. Informative, Educational, Topical, Authoritative and optimized very well for organic search so that others will naturally link to you and talk about your.
But what are some specific tactics/methods that you can do on your own for SEO/M?
I would say learn the basics then get your on-page optimisation sorted (Good start [google.com...] and then start thinking about how you can actively seek links. There is lots of information out there on SEO and the forums here are very active.
I would say though that SEO can feel a bit never ending - no one ever knows exactly how the search engines work but everyone has their own opinion. Try not to get sucked in too deep - just follow the guidelines and make incremental changes and be patient.
The one thing everyone agrees on is that links are important so I would recommend checking out the Link Development forum [webmasterworld.com...]