is not based on acquisition price of rental property. The rate of return is based on the amount of money you put in the property. The $20,000 home would not have required any more than 10 to 20% down payment or $2000-$4000. I bought homes like this back then and all of them were positive cash flow from day one with the amount of cash flow only increasing year by year. The return on the DOWN PAYMENT far outweighs the return on stocks that were purchased at the same time.

There are so many other benefits that you are not considering:
depreciation which is a paper loss, not real cash, to offset real income and reduced taxes on all rentals.
Cash flow generated that may be used to reduce mortgage or acquire additional investment property.
Principal reduction being paid by a tenant which eventually returns a free and clear home to you.
Ability to refinance over and over again and use all that money with pain zero taxes.
Use a 1031 exchange each time if you decide to sell the property and leave all tax consequences with future generations that have not enjoyed the monthly or annual cash flow.

Median stock market Return does not compare to rate of return on real estate down payments.