For the most part yes..

But how would you relate seo/marketing to future growth, i.e if you spend x on seo & marketing this will increase your market share by x (which is worth? in sales?) Is there any to gauge the potential someway accuratley?

Let's just say i'm going to start a hosting company, The Irish Market is worth 5 billion (how do I get that market value?)

I supoose my question is, how do you know the value of your market, does any company compile reports on Irish e-commerce trends & market value?