It varies so much, it's hard to say, esp. with whether the publisher is large or small (i.e., how much money it has in its coffers) and with the author's platform and reputation (a publisher might not want to offer an amount an author or agent would see as insultingly low to a v. distinguished writer).

But advances have stagnated for years for books publishers don't expect to be bestsellers. I've known authors who've still had $5k recently and some who've had as low as $2k. I've known others who've received much more.