I am learning this as I try to buy rental properties. I find my local area has many rental properties in few zip code which others (zip codes) have very few.
The rental price sweet spot is about $1000~$1500 in my local area.
you need to find out your local rental "sweet spot".
My rule is :
Do not buy the property that is too expensive (less tenant base) or too cheap (less cash flow for the hard work)