I have been struggling to understand how to calculate the rental income of multi-family properties.

I found a 3-plex (2/1.5 , 1/1, 2/1), and Rentometer is telling me 2 bedrooms can be rented out for ~$1500. Assuming ~$1000 for the 1/1, the monthly rental income for this property would come out to $4000.

Well I checked craigslist for the rental rates in the area, and luxury apartments rent for $1700 a month. I'm talking newer, bigger, cleaner, and prettier properties. Even if I rehabbed my deal, I can't fathom who would rent a room in a decent multi-family over a luxury apartment for only $200 more.

So is it fair to assume that the rental income should be much lower?Are there any other ways to figure out rental income besides calling a property manager?

P.S. I don't quite understand rentometer. It does not give an option for multi-family renting.

Wouldn't renting a multi-family be cheaper than renting a 2 bedroom house? (Simply because you are sharing a property)

Should the rent of a 3-bedroom unit of a multi-family be equivalent to the rent of a 3 bedroom house? (assuming same sizes/amenities)