How much to devalue a bike with rust/ needs paint?

I'm on my way to pick up a $75 1985 Schwinn Tempo, and on his own the seller just knocked the price down to $50 and sent a full disclosure shot of the sweat rust on top of the top and down tubes. I've pretty much decided to take it on as another project in the pile, and since its a black bike I'm more likely to be happy with a touch up job an a bike I'm going to keep for my self.
My more general question, Is how do you value or devalue a bike that will need a paint job to be "whole"?