Why are you so pessimistic? Just because something is hard and you get big error bars doesn't mean we can't do it at all.

If you wanted to, you could look at eg black market prices for kidneys to get an estimate for how much your kidney is worth. Or, less macabre, you can look at how much you'd have to pay a gardener to mow your lawn to see what the labour of your son is worth.

"I don't think we have one", not "I don't think one can be invented"