It might not be costly because Yuuko's prices always seem to have to do with personal value, and Kurogane does not really VALUE the water. He's only wishing for it in order to save Fai. However, if one takes into account that it IS important to him BECAUSE wishing for it will save Fai, then it might turn out to be a heavy price after all.
...I guess there was supposed to be a point in there somewhere.
Poor Fai, though. Even if it IS with Kurogane, he's completely dependant on this one guy for the rest of his life. He can't leave him, ever. That's got to suck, even if you love the person. Maybe it's just me (since I have...freedom issues...) but it would seem a bit overwhelming, at least at first. I wonder how often Fai will have to drink? Every day? Probably not, since Clamp wouldn't be nice enough to give us daily KxF necksucking fanservice.
XD