Hi Craig - most publications calculate fees by word rates. They might just offer you a fee, but you'll find that the fee is based on a word rate if you ask. Rates vary dramatically among publishers, and between publishers in different countries, but they're all driven by circulation figures (for print) and page views/readership (for web). Different writers also work for different rates and writers find themselves setting standards (i.e. I won't work for less than XX) and then dropping because they're passionate about the project/product. Having said all that, the industry standard for print works out to about US$1 per word for magazines, less for papers, although the better magazines pay even more. Last time I worked for National Geographic Traveller they paid $1.50 per word, while another magazine I worked for paid $2 a word. I won't work for less than $1 a word for print for UK/US/Aust publications, however, I'll work for less for Asian/Mid East publications. For web content, I won't work for anyone that offers less than .50c a word, and that seems to be the standard for quality sites. And then there's guidebooks, a whole different ball game. Hope that helps.
It varies really dramatically. As Lara says, a fee will usually be based on a per-word rate even if it's quoted as a flat fee.
A few examples though:
Australian newspapers/ websites/ magazines: The standard fee appears to be AU$0.50 a word.
Canadian newspaper (I've only written for one group, so have nothing to compare this to): Approx CAD30c a word.
UK magazines: Between GBP0.15 and GBP0.30 seems about standard for the non-luxury end of the market.
UK newspapers: For the nationals, GBP0.30 is usually the minimum.
Websites (proper ones such as AOL, Virgin Media, MSN - not wishful thinking start ups that pay feeble fees like $25 all up): Between GBP0.20 and GBP0.30 a word.
The bigger, glossier magazines tend to pay much higher rates, however - more along the lines of what Lara mentioned.
Personally, I generally stick to GBP0.20 a word or AUD$0.50 a word as my baseline, but there are a couple of publications I write for regularly that pay less. I keep them on because they offer regular work, and are useful for securing tourist board assistance (the editors get back to me quickly). Essentially, the piece for them keeps the cost of the trip down, and then I can sell plenty of other pieces elsewhere afterwards.
David does make a good point in that last para and I'll add to that, as we work slightly differently...
That's why I do guidebooks, for which the word count is considerably less than it is for the magazines, however, the beauty of guidebooks are the content-gathering opportunities they provide and the big lump sum fees - in advance, on submission, and then when the thing goes to the printer.
I then use the very helpful commission letters/letter of intro that DK/Rough Guides/AA etc give me (LP does not provide these, doesn't allow its authors to accept media rates and doesn't pay expenses - one of the reasons I no longer write for them) to get us complimentary or heavily discounted accommodation, car hire, etc - essentially 'media rates' like corporate rates.
I also snag a couple of commissions ahead of time. And then the guidebook research trip - anything from 6 weeks to 5 months (my latest) - enable me to gather tonnes of content on the way. Once the books are in I then pitch away.
Not many guidebook writers seem to work like this for some reason - and then they complain about the fees - but I know Kim Wildman does, so she may be able to add some more helpful advice.