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.