In that scenario, the guy will have all of the gold in TBC, and the same amount in Classic. (Keep in mind, he has to pay real money to have the character in both.)
But, there is exactly the same amount of gold as before in both BC and Classic Era.
Here’s a way to look at it. When the snapshot happens, they are effectively cloning the whole server. So, now you have two identical servers with the same resources on it. Imagine then that any character on the server someone doesn’t choose is now “locked” in that snapshot state and can’t be logged into. (Which is how they described it in the Q&A.) You can pay to unlock that character at any time, but until then, it’s in that locked state. So, as it’s unlikely that everyone is going to pay to unlock all their characters, some of those resources are now on “locked” characters unless someone pays to play the character in both.
So, any gold moving shenanigans will only work to maximize how much of the current gold remains active on each server vs. of lost on “locked” characters.