Alef:
My first, cynical reaction is that with so many good writers there are in here you could probably get your story for free unless your specifications are really strict and strange. (Sorry to ruin the market, guys!)
I've seen requests for general types of stories which end up being met (for free) by Library authors, however of course if there are specific requirements in terms of characters, plotline, setting, length, etc., then hiring somebody to write a particular story might work best.
Obviously, the client should check out some of any considered writer's works in this Library, to determine if that author's writing style works for the story which he/she is commisioning... --C.K.