Anna Louise at LiveJournal provides hard facts. The tag on this post is “demystifying publishing” and indeed, it is a very very detailed and for many probably very sobering account of how advances are calculated and where all the money goes.
My take on all this: I don wanna. I won’t even read my royalty statements. I call up my agent and ask for a three sentence summary/bottom line, and then I let all that go. In my case, too much exposure to Numbers results in a shut down in the writing process.