Per square millimeter, memory costs less than gpu silicon. (and Radeon VII's memory takes up more square millimeters than half).

https://www.anandtech.com/show/13923/the-amd-radeon-vii-revi...

(Also yield decrease per transistor added is nonlinear, so going from a 331 square mm to say 495 square mm would probably decrease yields by roughly half, even though number of transistors would be less than doubled).