74 + spare filter = 36 months of clean air if you get 20% off both at the checkout (there's' already a fiver off the main item) = ") GBP 20 off.
I do HEPA long-time (great if you get hayfever too); adding UV-C (not with ozone) ...
The maximum power drawn by the AR-10 is 38W; it highly depends on your energy supplier how much it will cost per hour.