Here's the truth of it:
If it's a NON-PROFIT organization asking for a trade, consider it a charitable donation. Feel good about yourself but be sure to get your name out there with a credit of some sort.
If it's a small FOR-PROFIT company, at least charge what it cost you to acquire the image. (Film, processing, gas, beef jerky...)
If it's for a LARGE, FOR-PROFIT CORPORATE ENTITY with yearly overhead costs that exceed the income of your family's last five generations, CHARGE A FORTUNE.
Why? Because it's probably IN THEIR BUDGET. Every print job worth being involved with will have a photography budget. By saving money on said budget by paying pennies, you're getting someone ELSE a promotion or bonus and screwing yourself and other photographers in the process.
This is the same "revolution" that happened to graphic designers and typesetters when the desktop computer came into existence. Everybody and their mother claimed to be a "graphic designer" and the consequence was qualified, talented designers were being outbid by younger, less experienced "designers" with a Macintosh in their parents' basement. (Albeit, the work was lower quality, but to some marketing guy that doesn't know the difference between a serif and a descender, who cares?)
Now, with digital cameras and Airliners.net, art directors and buyers are smartening up to the fact that they can get "decent" imagery without having to pay for it. Learning how to price your images is just as important as learning exposure settings... if not more so.
Yes, it's nice to see your photo all over the place, but keep in mind what you are doing. It affects the market and quite honestly, in some cases, you are throwing away a winning lottery ticket.