Give these guys a call (or visit their website):
http://www.networktechinc.com
I spoke to them on the phone...they will make custom cables for you and they guarantee their video cables to run up to 100 feet. I feel confident they could fashion an ADC extender cable for you.
Cheaper than Gefen....
Although if you need other things extended at the same time (USB for keyboards / mice), built in audio, etc....the Gefen box is still a good (although pricey) option.
Best of luck