There is longstanding legal precedent establishing in the United States that the "fair market price" is the price for which a given property would sell as negotiated between a willing seller and a willing buyer. That is established, in the absence of a willing seller, by independent real estate appraisals based on recent sales of comparable properties in the same area. There is no way under American law, pursuant to the Constitution, to get around that. The government has the right under the constitution to take the property at a fair market price, whether the seller is willing to sell or not, provided the taking is for a legitimate public purpose. I don't have to agree with that, but I have to accept that to be the case, unless the Constitution is amended to abolish eminent domain. That has never happened in earth's history since governments have been established. There could be no "public works" construction without the government having eminent domain rights. One property owner could hold up the government for far more than "fair market value" for his land unless he could be forced to sell at a price determined by others. There would be no highways, no railways, no courthouses, no government offices, no dams, no bridges, etc. There would have been no railroad for Dagny Taggert to own were it not for the railroad's ability to use the government's eminent domain powers to acquire rail right-of-way.

Or better than that, maybe he was a man of principles that he held in high regard.