Sharp filters are expensive to replace. The hepa filter is 100+!
All are around there. But yes I think Sharp filters carry about a 10-20% premium over others overall. But you cannot solely look at price alone.
You'd need to look at (1) size of the filter (2) the thickness of the filter. Both points 1 and 2 make up the volume. There is a certain amount of fine particulate matter than this volume can hold, the concept is not unlike a vacuum cleaner bag. The fuller this filter is, the more clogged it is hence the less airflow it passes. A bigger room has more air/dust in it, requires a bigger air cleaner which in turn requires a bigger filter. A bigger filter = more $$$.
Blueair filters are very expensive, like 389 if I remember correctly for the 500/600 series replacement. But you get 3 filters as a set coz the unit uses 3 of them, and each filter is bigger than than a Sharp A50 replacement (S$109 each).
At Best Denki, the Novita filter costs $89 which is slightly smaller and thinner than the Sharp A40 ($89).
Honeywell ones are hard to gauge as they are cylindrical in shape.
Nevertheless, using 8-12 hrs a day, I change my filters once every 1.5 years. Mine is the Sharp A40 HEPA-13 replacement filter which costs S$99.
This works out to be $5.5 per month. You'd notice a slight decrease in airflow at the 1 year mark and at 1.5 years it's quite noticeable (but still usable) drop. In the interest of efficiency and cleaning power, I'd then change the HEPA filter.
The concept is just like a dirty fan or air-con lah.