Is it valid to compare cryptocurrencies by using their smallest divisible unit?
No, the above calculations are not a valid way to compare cost of these curencies, for exactly the reason you state:  a cryptocurrency is just worth what someone is willing to pay for it.

They don't have intrinsic value. Cryptocurrencies are inherently wealth-destroyers, they are not wealth: they require resources to construct, and then just act as ponzi schemes thereafter. So their only value derives from the number and purchasing power of the people that can be fooled into buying them.