How much to invest to increase rent?

[David Mejia]

Hello guys. I have a duplex that is currently month to month with both tenants . I have recently reviewed the rentals rates in my area and seems we are currrently couple hundred below market. However, my units are not in the best shape ever because they have been rented for so long they have not being updated. My question is how do you determine if upgrading the unit ( kitchen , bath , floors) is a good idea when trying to get better rental rates ? 

Is there any startegy or ratio for how much to invest to increase rent? 

Thank you all