You are correct. Mises discusses this point in the Human Action citation above. The monopolist receives a capital gain on the price of his assets when investors at large recognize the monetary advantage of the monopolist’s position. Proper accounting of the higher price of assets would render a normal rate of return. The historical case often cited to illustrate this is Alcoa’s exclusive ownership of bauxite mines. Once investors at large recognized the extra profitability of such a position, they bid up Alcoa’s stock price reflecting their view of the higher asset prices of the bauxite mines owned by Alcoa.
Rothbard also discusses the “capitalization” of asset prices in chapter 4, section 4, sub-section b on Property Taxes of his book, Power and Market: