You know you can upgrade units, right? Build a rifleman, upgrade him to infantry if need be. UN costs the same as a single GDR. Once you've finished the tech tree? No. The game is done at that point, it should be ended. It is. I've never got to the GDR, I can win long before then, including by late-era conquest of any other civs (beelining their capitals). I agree that high end tech costs aren't high enough though. Science rates increased at a faster rate than populations do.