Older windows, fix or replace?

[Mark K.]

I have a triplex that was built in 1995.  It was abused over the years.  I bought it two years ago.  It took me until now, but I've gotten the inherited tenants out, renovated each unit, cleaned the place up, replumbed the building and more.  The one thing we didn't do was any work to the windows. 

i have increased rents from $600 to $900 now.  I have new tenants who have expectations that all windows will work properly and have screens.  

The windows have wooden frames.  

I had a maintenance guy there and he didn't really do anything.  He works for 3 property managers in the area and he said they don't replace windows unless they break, they don't replace screens and as long as it locks, they don't care if they don't open properly. 

If I wanted screens, I'd have to have new frames made, and I have no idea what they cost.

I don't want to be like most landlords in my area.  My question is, at what point with windows do you just put a new one in vs trying to replace it? 

Thank you.