This is really tough.
I agree 100% that link building contributes to the bottom line in a big way. The problem I have is assigning an ROI value to the different activities necessary for cultivating those links. For practical purposes, one cannot do the analysis paralysis thing but leap into it and assign an actual dollar value to the activities. But the actual value (when successful) is far and away greater than the money invested.
I posted the following on my blog a couple years ago, and I'll share it here:
The ROI of search advertising is directly measurable. The ROI of television advertising cannot be directly measured. The kinds of activities useful for attaining natural links are closer to the television advertising model in the sense that they cannot always be directly measured…
The impossibility of direct measurement is not a problem inherent in the process. It’s a problem imposed upon the process from without, by Internet Marketers and clients who are accustomed to measuring ROI and choose to make it a problem.
What I'm trying to communicate is that Internet Marketing is accustomed to assigning ROI values to activities and advertising spend. However the kinds of activities necessary to producing natural links tend to not have an immediate effect and sometimes they have an indirect effect.
Then comes the issue of valuing a link according to a third party metric that often has no correlation, zero, to influence for ranking. That's something imposed from outside, from without, from on top, like forcing an object with a square hole onto a round peg.
Then there's the consideration that some backlinks may not contribute toward a ranking, but might contribute toward authority, because the link isn't really about Blue Widgets so the vote doesn't carry any kind of ranking power, because the page the link is on isn't about Blue Widgets, but maybe it's about Widgets in part or whole. But how does one know which link contributed to ranking, which link contributes to authority, which link contributed at all?
It's really tough.