Sigma AF 10-20mm f/4-5.6 EX DC swap for D FA f2.8 100mm Macro

Hi,
I have a D FA f2.8 100mm Macro and someone is interested in a swap for their Sigma AF 10-20mm f/4-5.6 EX DC.

I'm trying to figure out rough values of both so we can decide if it should be Sigma AF 10-20mm f/4-5.6 EX DC & €X for my lens.

Anyone able to help?
I'm going to go through the FS forum but prices vary and someone might know off the top of their head.
Thanks,
Patrick.

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Cheers,
Pa
http://www.photoblog.ie where every post have a musical reference as it's title.