Lots of folks would and have argued its impossible to find "2%/50%" deals. In many areas, like Denver, I would tend to agree. But we do see deals like that crop here. There's even a group for that. So, it does appear than in the right location, you could really purchase properties for $25K and expect to get $500 in rent. With the 50% rule, you'll get NOI of $250/unit. So, you could buy four of them for cash with your $100K and be generating $1000 a month, $12,000 a year, in returns. That's 12%. Really, your investment would have to cover closing costs, so maybe you have $110K invested.
Alternatively, say you can get 70% LTV loans. Not easy for small loans like this, but say you can do it. Say its $2500 in closing costs and loan fees. So, you need about $10K each. Payment on a $17,500, 30 year, 6% loan would be $105 a month, leaving you $145 in cash flow. For the same $110K, you could buy 11 of these. That gives you $1595 a month or $19140 a year on your $110K investment.
Your total rent would be $5,500 a month. By doing your own property management, you could earn back the property management costs. These are all over the may, but 10% of collected rent would be a reasonable estimate. That's another $550 a month you can EARN doing the PM, or $6600 a year. That brings your total return to $25,740 a year.
Or, you could really put effort into finding properties you could acquire with little cash out of pocket. Subject to, lease/options, owner financing, etc. Build a good relationship with a small bank and work up to having a line of credit to buy and rent, then put them under a blanket loan. With a bunch of work, you could parley that $100K into a much larger set of properties.