I replaced my air filter recently, to a K&N. Primarily because i'm quite lazy and dont want to have to keep changing filters every 6000 miles or whatever the interval should be... it was £32 give or take. an OEM filter is somewhere in the region of a tenner, so assuming i do 12,000 miles a year, it would take me 18 months to recoup the cost of the filter.
But my MPG has gone up, from 54mpg to 57mpg. Again, assuming 12,000 miles per year and fuel at 129.9, that's a saving of £69.07 in 12 months... Or, across the same period of 18 months, £104.
So a £32 outlay for me over an 18month period saves £104 (18 months fuel, break-even on the filter)
50,000 miles (4 years) before the K&N needs servicing, in that mileage, i should save £287.78 in fuel, plus 8 air filter changes.
So if anyone is undecided wether to fit a K&N or not, hopefully the figures would suggest that it is worth the initial cost.
* i also replaced my tyres, which may also contribute to the mpg increase