I'm guessing the people who own those things acquired them with money. I'm sure it's possible to figure out a value.

Its not accurate unless you tax the sale price. An estimate could be off by 10s of millions in the case of $100m dollar properties.

I think one way of getting values that are reasonable is that if you claim something is worth X, the government should have the prerogative to call your bluff and purchase it from you for X * (1 + c). If you're making a reasonable attempt to value stuff, you should be mildly pleased with that sale, if perhaps inconvenienced. If you're trying to pretend stuff is worth far less than it is, you're at risk.