Thanks for the responses.
Stream of the Sky your recollection is largely correct. There is no cost as such to cast the spell once, just a percentage chance of the page in the Book turning by itself to a new and more or less useful spell. Eventually the Book runs out of pages and vanishes.
Doug, I like your calculation and between 30K to 50K sounds reasonable, so I think I'll go with that.
Stream of the Sky your recollection is largely correct. There is no cost as such to cast the spell once, just a percentage chance of the page in the Book turning by itself to a new and more or less useful spell. Eventually the Book runs out of pages and vanishes.
Doug, I like your calculation and between 30K to 50K sounds reasonable, so I think I'll go with that.
