@Braden Downs - It really depends on the market you're in. Brandon buys $80k houses and cash flows $100 per door. That makes sense.
If you're purchasing $500k houses, $100 shoudln't cut it.
Personally, I invest in Denver and I try to make $1,000 over the mortgage. This isn't quite the 50% rule. My mortgages have usually been around $2,000 per month and I am perfectly fine with $3,000 in rent.
Hope this helps!
Cashflow per door by itself is a tricky measure. It is to easily influenced by financing. If I pay all cash vs all finance. I like to look at the gross rent multiplier as well. It is one of the few "real" numbers, tax and insurance are the others. The rest are calculations and can be manipulated, maintenance, vacancy and so on.
The exact cash flow is going to hyper local. Rents have not been keeping pace with appreciation. This will also effect the cashflow.