0 down bank financed, what is a good return on investment?

[Jacob Carter]

I have an opportunity to purchase 2 multi family apartment buildings. 

12 unit with 9% cap rate, $400K and ~$110 per door monthly cash

8 unit with 10% cap rate, $300K and ~$110 per door monthly cash

Seems to be after all cost a ~3% return on debt investment. (Without management fees it would look far better... but I’m out of state.)
Is there a rule of thumb I can trot out to evaluate investments on a debt to monthly cash flow ratio. Like for every $50k invested you should see $100 in monthly cash flow to be in the ballpark of good. Does something like that exist?