I understood most of this intuitively, but I never thought about the "you might win it later" part for upped cards. I upgrade way too aggressively.
I just now reached a new personal record of 14,000 electrum, but only because WAR is coming up and I want to have upgrade versatility. Were it not for that, I'd probably have spent it all on upgrades I don't need for decks that I want to "try out".
Thanks kevkev.
Almost exactly my situation, AND I have another tip, adding on to what Kev said:
Unless your using the deck your upping to farm with at that exact time, don't start upping it until you can afford to fully up it. This means, even if your going to start farming with it game after next, and you only need to win 10 more electrum, don't even start on it yet, still a chance you can win one of the cards you need. For example, started upping day traitors while using instosis. Finished upping Rol's for it, and next game I won one. Easy waste of 300-something electrum.