Eileen, that's a trick question. There's no objective answer because it really boils down to how many copies the publisher thinks your books can sell. (Whether you are a first time author or not only has a bearing in the sense that if you are a previously published author, they have a better gauge on how many books they expect to sell.)
Erin Morgernstern (Night Circus) for example was rumored to have gotten a $1 million advance--but they expected the book to be a hit and there was a big marketing push for her novel.
If you want a more modest example, midlist authors (midlist tends to be copies sold in the five digit figures but not breaking the 100,000 figure barrier) get an advance of a few thousand dollars, at an average of $5,000. Tobias Buckell conducted a survey back in 2005 with these results, which might help you: http://www.tobiasbuckell.com/2005/10...ey-version-20/
(Oh, and a caveat, these figures I am quoting are for fiction. It might be different for nonfiction.)
I am confused by what you mean by commissioning. Under what context are they commissioning you to write the book? That's common for nonfiction, so I have no figures for that. Or is this for a media tie-in novel? Ghostwriting?
Depending on how much copies you think your book will sell, other options include selling it to an independent publisher (I'm using that in the sense of traditional publishers not under the big six, as opposed to self-publishers) or self-publishing it yourself, depending on your specific circumstance and projections.