You can buy the housing individually. I have enquired about it.
You just need to re-use your old bulbs/xenon ballasts.
Its around $350 for the housing, if i remember correctly. From VW Spares, that was trade price too.
Do you have a part number for that? I can't seem to track one down. My local dealer claimed they could only sell it in full.
If you've got a part number, I'll track it down. Can't see one on ETKA either.
I don't have a part # for you, but have found the housing at 1stvwparts.com - here's their description:
Description - Year - MSRP - Your Price
Headlamp assy Left - Gti - Gti 06-07 $420.00 $311.22
If you can't find the part #, perhaps this might work for you. Hope it helps you out!