Just wondering if anyone could give an indication of what kind of payment Publishers offer an author to publish a non-fiction book - I have no idea what they pay. I have to say I'm very curious!

I googled it but could only find a couple of examples:
The author if a quality history book which took 2 years to write got paid an advance of £12K. And Lynn Truss was paid £10K for Eats Shoots & Leaves.
An acquainance who writes non-fiction and can churn out a book in 6 months, puts down £4K-6K in his book proposals, depending on how much he thinks it's going to cost him in expenses to research the subject, copyright fees, etc, but I don't know what he actually gets.

<Added>

The examples came from here, which also gives a useful breakdown of book retail prices:

http://www.danutakean.com/blog/?p=46

I think it depends on lots of things, like the size of the publisher, etc ect. My friend who writes non-fiction got a large advance from Yale University Press and a smaller one from a smaller, independent publisher (Old Street). However, the one with the smaller publisher has already been reprinted (it came out in October) and had several reviews in major newspapers. My mate was also on Radio 4 talking about it, and has been commissioned by the BBC for something or other, so it's all swings and roundabouts.

My advances from non-fiction were never more than £2k and the cost of photo permissions and copyright fees ensured that actually, I made a loss! But I did them purely for pleasure as I really enjoy the research.

Like the others have said, it's an unanswerable question really - could range from 2k to 200k, entirely depending on the publisher, the type of book, the perceived demand for the book in question, your expertise and track record, etc etc.