The primary perk for a more expensive unit is the ability to modulate that fan speed based on the particulate level in the room, IMO. I've got a couple of units

The primary perk for a more expensive unit is the ability to modulate that fan speed based on the particulate level in the room, IMO. I've got a couple of units that I paid ~$140 for which can step their fan speed up and down based on the measured air particulates. Most of the time, they're running at low speed and are effectively silent - just cycling the already-pretty-clean air in the room at a low rate - but if something causes a spike in particulates (say, cooking), they'll crank up to a significantly higher speed (and level of noise) until the air is cleared. If you've got a box fan running all the time anyhow and don't mind the noise, though, strapping a HEPA filter to it makes plenty of sense.