There was an episode where Dream sought the help of Lady Johanna Constantine to retrieve the head of Orpheus. Lady Constantine told him that the price of helping wouldn't be cheap. What was the price/prize Lady Constantine asked of him?
I believe it is not stated explicitly anywhere. My theory is that when Dream said
I will give you what it is in my power to give you
Lady Johanna accepted this as a general favor owed by a very powerful being, but ended up not using the favor while she lived. When she turns over Orpheus to the priests in charge of guarding him, she seems keen to see him again and suggests that she might return. Orpheus replies,
I do not think that would be a good idea, Johanna. Goodbye.
This is followed by her monologue,
I never saw him more. But, as the years have passed, I have, on occasion, seen him in my dreams.
And according to Wikipedia, in the "The Dreaming" spinoff, there is an episode where
Hettie [...] bribes Johanna for her silence, promising her that she would live to age 99. This promise proves true, with Johanna dying at age 99 while getting out of her wheelchair when she hears the song of her old companion, Orpheus.
Finally, in the first chapter of "Brief Lives", we see that Johanna is buried on the island where Orpheus is guarded by the priests, and the priests place a cherry blossom on her grave every spring day.
It seems that this is what Johanna (possibly after her death) asked from Dream - a wish only he could grant.