In part, this is because the initial resource costs of GBs go up significantly – by nearly a factor of 3 for each age. It’s difficult to figure out exactly how much though, and whether the difference is fair or not. On reviewing their benefits, it is clear that some ARE better than others.
In effect, they are like large Premium buildings (often much better!), which makes sense given that many were at least partially bought with diamonds, and/or require a LOT of investment to complete. Great Buildings are definitely better and more interesting than regular buildings.
