@Lowe's B&M Keen Smart Vent (ZigBee) 10-in x 4-in and 12-in x 4-in for as low as $14.25 depending on where you live.
Lowes 12-in x 4-in:
Lowes 10-in x 4-in:
These will work with SmartThings, Iris, Nest, Ecobee, Wink (not certified), and VeraPlus (not certified). The Keen Smart Vent uses Thread, so it might also work with Google Home and Alexa.
I personally tried these with a VeraPlus and they work very well (although the anemometer feature is missing with VeraPlus).
Again, YMMV (worked out very well for me).
Brickseek is showing $56 at a store I purchased the 12-in x 4-in for $14.25... probably would not use brickseek to determine stock or pricing for this item.