Maybe % of purchase price (though that would mean low estimates for killer deals, and higher estimates for hot market properties)?

My concern here is that depending on where your property is located, that might not be the market standard in that neighborhood.

Quote from @Eric Lahoda: It depends on the market and your skills as a landlord.

Hopefully along the way your properties appreciate, you pay down debts  (mortgages) from operating income, and you get market appreciation on what you can rent each unit for - giving you more income each month. 

Made good money for a while then the market turned and he lost a lot of coin ...  

I know the market fluctuates and that can be a factor, but generally speaking.

Most buyers don't have the money to come in with to pay down payments, plus commissions in this market, so I don't expect a lot to change.

Starting out in real estate with a focus on Section 8 rentals, the BRRR method, and Airbnb properties is a strategic approach.

I know Austin; I don't know other markets.

They may come up but understand that rental rates are generally well below market (hence the discount) and these deals are largely off-market if you're able to find them.