Nothing wrong with the analysis the problem is that they are going off the sellers information. I have seen duplexes listed that are at 120k, rental income 900 per month and the owner pays utilities. There is no way with any kind of debt service that you are in a break even on the investment. The owner bought it 12 years ago for 40k and it is most likely paid off. For the current owner it is a cash cow for a new investor the cash only flows one way. If you hear that big sucking sound that is your cash reserves doing the cash flow thing.
The listings agents job is to get the property sold not to do your DD. If "cash cow" gets your attention then it has helped to market the property. Never trust the numbers provided by the listing agent when you are looking at investment properties. Partner up with an agent of your own that can run the numbers for you. You would preferably want an agent that owns rentals also. Not that manages them for other people but has there own hard cash invested in a property and deals with their own tenent issues.