And some big, big authors talk quite freely about advances, contract terms, etc. on writers' loops, so it's not that it's not done.
I was going to say something similar. There are some major names that I've heard get pretty specific about numbers at conference workshops, if not on websites. Not to the penny, but ballparks, like the publishers lunch type of thing that reports "deal" and "nice deal" and "very nice deal" or whatever the categories are. The number of zeroes in an advance and the number of books it's for, and what, if anything they expect to earn over the advance.
I'm sort of up in the air on whether to do it in any more detail or in a semi-permanent venue (compared to a workshop that might be tape-recorded, but only in limited distribution and the tapes stop being available in a few years).
I think it's good that writers are getting together and talking money (as in Brenda Hyatt's "show me the money" website, and RWA's got a "second sales" column that gives ranges of figures for sales without giving specific names), so we're not quite so timid and accepting of the least little crumb that's offered. (Not that my publisher offers crumbs; I'm happy with them, and they met/exceeded my expectations, so this isn't personal.) I see too many writers -- and women particularly, and this is true in the folk art realm, too, where I have experience as a quilter -- who have the attitude of "I love what I'm doing, and any money I get is gravy, so I'll take whatever's offered and be grateful."
Wait, wait, stop me before I head into that whole rant and forget the original question. Except I don't have any good ideas for you. Definitely not legal advice, b/c this is outside my expertise/jurisdiction. But not even professional advice. I've auditioned a dozen different answers, and keep coming back to "I dunno."
JD