You've got to consider the following things, which greatly reduce the actual "income" you're getting out of them. The obvious ones are mortgage payments, insurance payments, and tax payments. Then you've also got a vacancy allowance, because you assume that units will be vacant at least some of the time between leases. This can vary between 5-20% depending on where you are.

And lastly you need to set aside money for repairs and maintenance. Most people forget to factor this into their calculations when thinking about rental cost. It's common for people to say "Well I'll just pay for it when it happens, and take it out of that month's rent". But that doesn't really apply when you're talking about things like putting on a new roof, installing a new furnace system, or any large "maintenance" project that's going to likely need to be done over the ownership life of the property. My Dad just put replaced the roof on one of his rental properties for the 3rd time since he's owned it. (He bought it when he was 21, is now 67, and the roof needed to be replaced shortly after he bought it.)

So yeah, for a single unit (depending on the area, again), you're doing well if you're getting even $100/month out of it. And as mentioned above, you're constantly building equity.