The Sigma 10-20 apparently works down to 12mm on a full frame (may have to check in terms of quality - i'm thinking lots of CA and distortion with a bit of vignetting...).  I have it for my 30D (1.6x crop) and it is a good lens - check my uploads from Westonbirt - they are all taken with it (so far).

But yeah - the here and now - good lenses dont tend to loose too much value second hand imo...

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography