Exchange what now? Just by Yukiko's expression, she never heard of the idea. A persona user does sacrifice a bit in order to use their other selves but the problem is that Yukiko doesn't see it as a problem, just something she does.

"I...honestly don't know. I never saw it as anything bad, just what was right. Other than getting hurt if our personas do, I don't see much of the exchange."

It doesn't mean that there isn't a cost, just that Yukiko didn't see it as one.