That's an awful solution. Some small fraction of people in the way shouldn't get an enormous multiplier over market value, in some kind of giant prisoner's dilemma auction.
If you offer the group a certain percentage of market value, that could work out well. But unanimous consensus is not a reasonable way to get land for big public projects.
“Market value” requires willing participants. If a seller doesn’t want to sell at a particular price and the good isn’t fungible (which housing is not), they aren’t getting market value by being forced to sell at a price determined solely by the buyer.
If you think it's impossible to assess the value of a property, the whole legal world disagrees with you.
And yeah, the point of eminent domain is to force the sale on unwilling participants. If used sparingly and without discrimination, it's a good power, and part of living in a community that will undertake community projects.
* the underlying assessed value, excluding artificial caps like prop 13
Sure, go with “taxable value”, but let’s not pretend it’s market value any more than the market value of a human life is what an insurance company values it at.
Well market value plus some premium for inconvenience of not being able to choose sounds fair, perhaps FMV + 20% or perhaps up to 30%. These are large amounts of money so perhaps the premium should be an absolute not percentage value.
FMV +10% for tolerance of estimate of FMV + 6 months average salary in the area would be generous enough to recompense the hassles of relocating.
Remember surrounding society, that is hundreds of thousands of other people, benefit from the infrastructure being developed.
If you offer the group a certain percentage of market value, that could work out well. But unanimous consensus is not a reasonable way to get land for big public projects.