How do i determine rents in an area?

[Yaniv Yarkony]

Im about to buy a duplex and when i use rentometer to verify rents it states that the average rent is $2700 for a 3/2 but duplexes with the same br count and sq feet size for some reason rents out for 1700-1800. So i went and checked numerous in the area they are all the same. But i just dont know if thats considered "under market" and if its worth to BRRRR those if they are decently renovated?
and how much should cashflow be? Is it ok to be in a negative and pay 700-1000 on the mortgage or is that considered a bad deal?

Thanks