Well, I've spent the past few years trying to find this book which I read as an early teen, but funnily enough, the title 'Bargain' is about as useful for searching for a book as when you're cruising ebay for music by that great Krautrock band 'Neu'.
Anyway, the general plot of the book is that a band of squares meet the devil, in manager form (as is usual) and sell their souls to him. Their performances change from boring lite-pop to Alice Cooper style theatrics (without knowing where the flowing blood is coming from, etc).
I would love to get a hold of this book again, or even a scan of it. The binding was like all kids/young adult books of the early eighties. Even the name of an author would greatly help.