Well,
I have just been playing for fun. Basic deck is earth with several expansions that could fit. Say, 90 upped cards.
I don't know what to buy now, other than a whole new rainbow deck. I can't afford that, though it would not be far away if I played. I saved some electrums. My earth ideas were dead ends. That is, nothing was any better than one main deck I am using.
So, I'm actually quite fed up with it. After endless grinding I would have 2 workable decks. That is not a lot of variation, and I'm not crazy about the opponents either. The high AI levels basically cheat, so those are really unfair games.
With the current costs, there is really no way to gradually add some upgraded cards and see how it goes. You should really collect a load of electrums, check in the trainer IF that idea works, and if it does, well, buy the whole deck. Sucks the fun right out of it.