Hundreds of gems over multiple turns, maybe...
The reason you didn't get much out of the Nexus was that Cor2 had to hold back on doing anything that required gems. Now that yours is down and his is up, he can freely spend all those gems that he has been storing up for so long.
But yeah, I am kinda amazed how Cor2 thinks that he has only a slight advantage in this game. Ever since you're the only two nations left, Man definitely has a lot of aces on his side, what with the elemental queens, Tartarians, unique items etc.