Some of the rare examples of software security bugs where the attacks are practical but not free: Breaking RC4. BEAST. DDoS. The rare arbitrary code executions that require significant time to attempt due to needing to spray the heap just right.
But most of the time, it's click -> own.
The other thing that prevents useful quantification is that we don't have a Gaussian distribution to be seen. Rate of discovery, maybe, but that's it. We do not have Gaussian distribution on size of protected items, density of vulnerabilities, or anything else. Without that most of our good risk estimation techniques don't produce numbers that mean much to managers.
This is, IMHO, the fundamental reason why infosec doesn't get listened to. Our management structures are still highly Gaussian-biased (decades of manufacturing envy). They can't even wrap their head around software engineering schedules and costs after several decades, and infosec risks have even worse distributions! It is, in my very strong and considered opinion, perfectly rational for infosec to be vigorously ringing the bell. Consider exactly the bug we're talking about now... it's a vulnerability that has the potential to destroy the auto industry, were it properly exploited. (It would not just by Chrysler caught in the crossfire! And my use of present tense is considered. I have little reason to believe this is fixed, nor that it is the only such bug.) Our risk management procedures can not even properly express the idea that an engineer drawing a line between two nodes in the car's communications bus could have that result, can not properly express the Black Swan nature of that decision. But that's not infosec's fault, or at least, not infosec's fault alone.
And it's certainly not a solution for infosec to get more realistic about risk, because "more realistic about risk" probably means we ought to be a good factor of magnitude or so more strident, not less! The more I learn about the risks in this world the more horrifying it gets, and it's getting worse, not better, as things get more interconnected. Again, look at this car bug... it's something that could not possibly have existed 20 years ago. None of us, from CEO to engineer to QA, have even begun to properly process what this means, let alone react to it! And we won't until the Black Swan bites and probably not even then!