Land barons don't set the prices. Linden Labs sets the bottom price for new land in the auctions, and residents set the limit on how much they will buy for on the grid. In reality, if you look, land barons are selling 512 for less than 5k in most cases. Most land being sold by land barons is being sold at the same price it would go for in the Linden Labs auctions. If you are looking at someone selling for 10k-20k for a 512, chances are they aren't a land baron. Land barons that don't exploit only survive by pricing to sell. If it is at a price no one will pay, then it is not a land baron that will last longer than a month. Land barons skim off the money lost by those that are impatient and want to sell fast. They aren't skimming money off of prices no one pays. If no one pays, then there is no money to begin with.
And to this I must give kudos to Ansche Chung. Her land purchase prices are actually astonishingly reasonable for the amount of land you get. I have seen some 'land barons' take over whole chuncks of sim and selling 512 plots for 10K, where as I have in many instances seen Ansche Chung selling a hefty 2048 plot for just over 9K.