I am a small-time landlord. There's no "marginal cost" to determine how much to charge for rent. The only thing I use to determine the price is the current market rate. And all I have to do is open up Zillow or Craigslist and do a search on similar properties with similar characteristics. It only takes ~5 minutes of research to get a competitive market rate. While RealPage might command 80% of the market for this typ…
Those statistics don't tell us enough to see if this is a problem, though. In the extreme case (obviously this is not true), everyone uses pricing software, so RealPage's 80% is 80% of all dwellings, and those 12,000 clients own 4.2M multi-family dwellings.
Also what matters is number of units, not number of multi-family dwellings. Owning a duplex does not give you the same clout over local rentals as, say, owning the only large (at say, 500 units) apartment complex does.