If you write down all we can know about pleasures (in the moment), and all we can know about pains, you may find parameters to compare (like "intensity", or amount of neurotransmitters, or something else), but there would be no reason why people need to choose an exchange rate corresponding to some measured properties. I believe your point is that we "have reason" to pick intensity here, but I don't see why it is rationally required of beings to care about it, and I believe empirically, many people do not care about it, and certainly you could construct artificial minds that don't care about it.
Pleasure is not what makes decisions for us. It is the desire/craving for pleasure, and there is no reason why a craving for a specific amount of pleasure needs to always come with the same force in different minds, even if the circumstances are otherwise equal. There is also no reason why this has to be true of suffering, of course, and the corresponding desire to not have to suffer. People who value many other things strongly and who have a strong desire to stay alive, for instance, would not kill themselves even if their life mostly consists of suffering. And yet they would still be making perfectly consistent trades within different intensities and durations of suffering.
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 ...
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!)