How common are advances in the LDS publishing world? And how is an advance calculated?
LDS advances are less common than in the national market. Some publishers never give them, others give them according to their own set of criteria. Some only give advances after the first book. I can't give you exact numbers. We usually offer an advance in the three digits. Sneer away, but it's all we can realistically offer.
Some publishers have a standard advance they offer based on the type of book—fiction might get $X while non-fiction gets $Y. Others offer an advance based on the number of books they expect to sell in the first few months. This is all guess-work on the part of the publisher because the advance goes into the contract before they get pre-orders and reviews back.