I personally think that the upcoming Keen Home vents are the best product for the price at the moment given the fact that they include a pressure sensor to avoid damaging your HVAC in case of excessive air pressure in the vents (if you close all of them for example). They are a bit expensive although there is a special pre-order discount going on right now, see this thread:
Bye for now.
P.S. You can contact Nate from Keen Home if you have additional questions about smart vents.