There are 13 LDS novels that have sold over 500,000 copies. I can't remember all 13, but I think they're a combination of Work and the Glory, Children of the Promise, and Tennis Shoes among the Nephites.
In general, however, a good strong book by a well-established author will sell about 30-40,000 copies. Most novels sell somewhere between 5-10,000 (which is where On Second Thought is right now--about 6500).
So really, volume is the only way to make a living. The two I have this spring are 1) the first novel in the New Order series, currently titled AWAKE, and 2) a non-fiction humor book for Father's Day. Covenant asked me to work on this, since there is very little available for Father's Day (compared to Mother's Day) and I've been going nuts to get it done. We only started talking about it last month, and they'd like it finished by the end of November (granted, it's short--about 20,000 words). It's going to be a very busy autumn.