It depends on some major factors that have already been covered: class of the building/area, payment and behavior history of the tenants, how much works needs done to the units if they are vacated, how much you can increase the rents for new tenants, how quickly the units will rent/how hot the rental market is in that area, how much you do or don't mind having to find and place new tenants.Sometimes I'll get a multifamily with GREAT tenants in them that are paying market rent, or maybe just a hair below, and they've maintained the place very well.

My only problem has been a commercial space where a hair salon had to shut down due to covid and I let them out of the lease. 

For instance, what if you had a fee that said that people with brown hair had to pay an extra fee. 

In terms of legitimate numbers, besides the construction cost above, remember that the majority of these homes are very old and adding rooms can be tedious, stressful, and might make you want to pull your hair out!

That is the difference between companies that want to try and make everyone happy and companies that are laser focused on their ideal client.  

Hiring Guido to drag them out by their hair?  

I brought a laser thermometer with me and pointed it at the ceiling vent.

Not only that, my hair and and clothes and all of my things smelled like smoke.

I'm ready to pull my hair out because its been so much hand holding and nagging to get people to follow through. 

It actually forces him to be laser focused on who he deals with.