The way I look at it, if your driving environment is clean enough to allow the use of a low efficiency air filter, you would only have to change a paper air filter every 100k+ km anyway. So it would take a very long time to see a financial payback.
On the other hand, if your driving environment is dusty enough that a paper filter would need to be changed, say, every 20k km, a low efficiency filter could result in significantly higher engine wear. The K&N filter would pay for itself quickly in this case, but it wouldn't be worth the increased wear.
If the driving environment is not dusty, but the filter clogs quickly due to large debris getting crammed into the pleats, a K&N might make more sense. This is what happens with the filter in my car. My solution was to install insect screening over the intake to prevent the big stuff from getting to the filter. Previously, it got pretty clogged up in 30k km. With the screen, I expect that I can easily double or triple this interval.
