I forget the exact numbers, but yeah, you can win an awful lot of electrum this way. The problem is that the 500hp deck doesn't have a very high win percentage. It loses a lot, though when it loses it usually loses extremely fast. When it wins, it takes awhile, but you will win about 900-1000 electrum in platinum league on a 500hp EM there. If you're lucky enough to beat a top 10 deck, you can get even more; I once EMed a high ranking deck and won something like 2100 electrum, and I had a few other games where they weren't EMs, but still won a lot of electrum.
The other problem with 500hp EM is that it's not that fun to play most of the time, unless you've found a different version than I used. I used it for awhile when arena first came out and it's really good for grinding score, but you will win very few rare spins because the win percentage is low. It also has the slight embarrassment of losing sometimes to 1hp decks :p On the plus side, it doesn't care if your opponent has 200hp for exactly the same reason. (I'm assuming your 500hp deck wins by deckout, like mine did)