The windshield frames seem to go for about $150 to $200 on eBay.
I've only seen one "bad" windshield frame, and it's the one in my '46 1/2 ton right in the center bottom.. not sure if that's where all of them go bad or not.
Looks like water collected in the bottom of the frame and it just rusted. I suspect the pressure of the crank pushing out helps "blow out" the bottom of the frame.
As far as why they sell for a decent amount, i'd guess it's because it's one of those "must have" items. They're not any more rare than the trucks themselves.. and the windshield frame from as far back as 1939 will work in a '41 thru '46 truck.
