One way: consider how many Windows users would switch OS's or not chose a Windows computer as their next purchase due to the lack of multi-monitor taskbars. I don't need any sort of study to say that it's probably not a very high number.
You want a cost/benefit criteria? How about (number of people who care) * (how much they care) instead of just (number of people who care)?
"Part of the hard job of product design is deciding which 20 features you have the resources to pursue and which 180 to leave for next time. If you choose the right 20, people will say you're the best new UI feature in Windows, or even the best Windows 7 feature, and they won't mind too much that you didn't get to the other 180."[1]
I think single features can certainly sway a purchaser. If I like the OS X notification center quite a bit and its integration with 3rd party applications I use a lot, then it may well cause me to lean in the direction to purchase a MacBook over a Lenovo. Of course they're all taken as parts of a whole, but it's still conceivable that people may not be able to perform their job/task in a way that they are accustomed to without X feature.
In addition, while some features may not sway you, it is not unlikely that you are an exception to the rule.
[1] http://blogs.msdn.com/b/oldnewthing/archive/2010/05/31/10017...