My general point is that whatever property you rely upon to make comparisons within pains you can also rely upon to make comparisons between pains and pleasures.
It seems to me that you are using intensity of desire to make comparisons within pains. If so, you can also use intensity of desire to make comparisons between pleasures and pains. That “there would be no reason why people need to choose an exchange rate corresponding to some measured properties” seems inadequate as a reply, since you could analogously argue that there is no reason why people should rely on those measured properties to make comparisons within pains.
However, if intensity of desire is not the property you are using to make comparisons within pains, just ignore the previous paragraph. My general point still stands: the property you are using, whichever it is, is also a property that you could use to make comparisons between pains and pleasures.
My general point is that whatever property you rely upon to make comparisons within pains you can also rely upon to make comparisons between pains and pleasures.
It seems to me that you are using intensity of desire to make comparisons within pains. If so, you can also use intensity of desire to make comparisons between pleasures and pains. That “there would be no reason why people need to choose an exchange rate corresponding to some measured properties” seems inadequate as a reply, since you could analogously argue that there is no reason why people should rely on those measured properties to make comparisons within pains.
However, if intensity of desire is not the property you are using to make comparisons within pains, just ignore the previous paragraph. My general point still stands: the property you are using, whichever it is, is also a property that you could use to make comparisons between pains and pleasures.