this feels like a 'dose makes the poison' scenario. you're talking about maybe $10 of difference annually if both were run continuously. and that's only if you bought an inefficient air purifier - if you elected to use, for eg, four Arctic P14 PSTs in a CR box then that's maybe double the kWh. this ends up being a couple of bucks more annually for the added benefit of having better air quality
I think $10 is too low an estimate.
An AirFanta 3Pro on low (2) is 5W, which is $10 on average (but more like $15 in high $/kWh places like MA and CA).
Some people are paying north of 0.50€/kWh (after tax), so this amounts to 22€ annually.
The point stands. If you care about this, it's a personal choice whether that comfort is worth ~20€. I would wager many people say it is.