Imho I think @Jacob Pereira follows the closest guidance I provide to my clients. Everything is relative. $100 to me 10 years ago meant a LOT more then it does today. But to someone else $100 could be food for their family for a week today. With that you have to set your standards personally as compared to your own goals. What is your personal freedom number? Is it $5000/month in cashflow? Then at $100 a door you need 50 rentals. At $500 a door you need 10.
With that knowledge $100/door might be possible in my city but if know I can go 2 hours south and get $500/door. That could potentially be 1/5 of the time (generally speaking) for me to hit financial independence. From there I have a decision to make. Do I stay local and grind to 50 doors or do I set up my schedule to make the commute to and from my investing market possible.
My point is you have to set goals. Run the numbers to see where you want to be. Then analyze the market to see where will get you there the fastest. The dollar amount per door is irrelevant compared to your overall why for being in the business. If its financial independence then just replace the numbers above with your own ;) .
Good Luck!