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.
I've always been more of a theoretician, but it's important to try one's hand at practical problems from time to time. In that vein, I've decided to try three simultaneous experiments on major Less Wrong themes. I will aim to acquire something to protect, I will practice training a seed intelligence, and I will become more familiar with many consequences of evolutionary psychology.
In the spirit of efficiency I'll combine all these experiments into one:
She's never seen Star Wars or Doctor Who.
She's never seen David Attenborough or read J. L. Borges.
She's never had a philosophical debate.
She's never been skiing.
Never had sex, never been hugged or even been licked by a dog!
She has so much to look forwards to...
(Though she'll be very boring for several months yet!)