Well, Pak40 is 120 points, cheap Sherman is 190. So a gun is 2/3rds of a cheap, full tank
More importantly, even if you get one tank out of every AT gun that engages your purchased AT guns do not all engage. Some are out of way of the chosen attack path, some are discovered by infantry before they can shoot at a tank.
As long as we can't boost the gun with better cover and concealment in fortifications I think pricing them more at a factor of 0.4 or maybe even 1/3rd would be in order. It is all about actually getting the historical benefit out of the guns.