> When you talk about "organic" links do you mean non-advertising links from other websites like blogs/news sites?
Correct.
> It seems like you get diminishing returns on SEO.
I agree, better to spend the money on ads than SEO consultancy.
> What's the point at which you'd recommended a site stop SEO and try other means of maximizing exposures. What would these other means be?
I consider SEO at this point to be about breaking even with the search engine bots: if your _don't_ do it then they might actually penalize you (e.g. your site is slow, you have broken links, broken redirects etc.), but if you do optimize then they will take benign view of you, but not necessarily a favorable one.