Hanauer, founder of Seattle-based public policy incubator Civic Ventures, said that valuing illiquid assets — such as startup equity or artwork — is a “logistical nightmare” and has “failed every place it’s been tried.”
Here's one proposed way to do it (not my idea): Ask the owner of the illiquid asset to declare a dollar value for it.. with the added stipulation that the State reserves the right to buy the asset at that dollar value. Thereby, the owner is disincentived to declare an artificially low value to avoid taxes.
This approach is not without problems, but it would tend to result in a more realistic market value for each asset.