I think that if in endless mode, the enemies' HP is scaled to your level, so, too should the xp and gold earned. Just a thought. It seems a little frustrating to work harder for less reward. Back to making goblins into porcupines!
In all seriousness, though, the computer opponent's blocking is flawless, though his offensive logic is flaky. There are times where he (she? it?) will completely miss a winning move.
You should have stopped getting talent points when your tree was maxed out. I'm currently looking into this.