Talking to our engineers here at work, they've purchased +100 foot VGA cables before. We're an architecture and engineering company and we've done a considerable amount of high tech work in the past and I'm fairly confident my guys know what they're talking about. They said that the cables do come at a premium because of the amount and quality of cable shielding required to run distances that far. The cable shielding is the main component that reduces "ghosting" associated with running VGA long distances or near sources of electrical interferance.
Your best bet is to buy the highest quality cable at the length you need. Anything over 25 feet, you're going to pay some serious $$$ for.
techfuzz