It's simple maths that you will save money with a K&N.
The Haynes Manual for my FZ6 recommends replacing the filter every 25,000 miles.
So that's 100,000 miles before you break even.
Simple maths.
I wouldn't leave a filter on any bike for 25000miles and I would be surprised if Haynes said 25000 miles since they take their information from the Yamaha manuals and the Yamaha manual states replacement at 12000miles 19000Km. See Picture
I know this because they Haynes technical support told me when I corrected a mistake on the 1999 FZS600 Wiring diagram, I contacted them and told them about the mistake in their wiring diagram.
The FZ6 Yamaha Owners and service manual state 19000Km and Haynes would have no reason to deviate from it.
The FZS600 run rich as standard because the air filter is more restrictive than it need to be and the reason given was to reduce the intake roar to pass the noise test.
I have fitted a K&N filter to all vehicles I own and have owned and never had an issue with them. In my opinion for whatever it's worth I would recommend fitting a K&N one to the FZS600.
A further issue I have come across more than once is that
some, but not all HiFlo Air filters were more restrictive than OEM and causes issues with the running of the FZS600 after after 5 or 6000 miles on the filter.
The more restrictive HiFlo filter would cause the FZS600 to die around the 7 to 8000 rev mark and pick up again when opening the throttle quickly, but didn't happen when going through the rev range normally.