You know better, even if it's just a hinky feeling that you have that brings up the hairs on the back of your neck.Once you take money from this tenant, you owe them.  

The chart above says Rustic Investments bought 33 liens for an average of 14.94%.

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. 

@Randy D for revenue industry standard is charging a flat fee (see chart below) or a consistent percentage of the monthly rent(8-10%). 

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!

I have spent many hours setting up a Chart of Accounts, watching tutorials, coding accounts, trying to get transactions entered to show up elsewhere without messing up some other account, and a host of other banging my head against the wall activities.I have found that Intuit is not the most. . . hmmmm . . ."

Hiring Guido to drag them out by their hair?  

Stop by on a Wed or Thurs and the guy that teaches the class even wrote a book he'll give you for free detailing the whole process with charts, graphs and actual conservative examples of how practical this strategy is.