To directly answer: I'd be looking in the range of the gross rents monthly being 2% of the purchase price (i.e. it should still be a good rental under the assumption you don't live in one of the units. Then moving into one of them is fine.)
This type of question leaves so much to be desired though. There is a lot of individualized analysis that needs to happen. Location, quality of tenants, cap rates in that particular area, quality of building, current occupancy, etc. etc. etc.