[ChrisRabior]

Check Amazon, a good starting mark for used gear is 60-75% of the current retail cost.

Quick story:
I wanted to sell my Canon EF 75-300mm f/4-5.6 III, put it up at a pretty major discount, and still got low ball offers. Couldn't get what I wanted for it, decided it would serve best as a backup.

With a 40D on order, I'm guessing my 75-300 and 18-55 are going to be on my XT while my better glass is on my 40D.. and my fiance and I can both shoot with the same focal length. Of course, if we're not using the same focal length, those lenses won't see the light of day =)

Set a price you'd be comfortable with. I don't know the conversion, but in the US on Amazon, your lens is $200. A reasonable price would be $100 to $150, depending on condition. If it's in the pristine condition that I keep my lenses in, I'd say $150 would be a good starting point. Basically 50% to 75% of the current retail price online, depending on the shape the lens is in.