There's a fair-sized philosophical literature on the possibility of incommensurable values. According to some accounts of what it would mean for a value to be incommensurable, such values would represent a failure of a common currency.
Joseph Raz, for example, says two values (or bearers of value) are incommensurable if it is false that “either one is better than the other or they are of equal value” (Raz, 1986, The Morality of Freedom, p. 342).
I'm not - for now - taking a position on whether there are indeed incommensurable values. It's just useful, either way, to have some philosophical work clarifying what it might mean for values to be incommensurable at all, and considering some arguments against and in favour of the possibility. I'll say more about some of the arguments in the future.
If you want to look at a clear overview of the issues, then Nien-hê Hsieh has written an entry on Incommensurable Values for the Stanford Encyclopedia of Philosophy.