You could start by valuing it at the price someone would pay for it, plus the price the worker would have paid for that many hours of hands-on education. This

You could start by valuing it at the price someone would pay for it, plus the price the worker would have paid for that many hours of hands-on education. This isn't totally farfetched: in some countries if you own your house you pay tax on the rent you would have paid to yourself [0]. Taxing someone for feeding or educating himself seems like a small and natural next step. [0] https://en.wikipedia.org/wiki/Imputed_rent https://en.wikipedia.org/wiki/Imputed_rent