I don't know how she figures it out, but it always seems like just the right price for the wish. Never too much, never too little.

Like, for example, one time someone wished to not be tied down by the relationship she had with her twin sister. So the price she had to pay was to cut her hair. Which worked out in the end, because once she had shorter hair it was much easier to tell them apart, and she started to form her own identity instead of being 'one of the twins'.