I don't find AI5 ever gives worthwhile rewards. You get basically as much cash from grinding AI3 (or even top50), and the chance of winning an upgraded card is tiny compared to vs. false gods.
Unupped decks can win often enough vs false gods to make grinding them better than grinding AI5, even though your winrate might be something like 20% vs gods compared to 80% vs AI5 (as long as you know which gods are hopeless to fight, anyway--you just quit those matches immediately to cut your losses. I quit immediately against Divine Glory, Rainbow, Scorpio, Obliterator, Seism, Hermes, and Graviton. I also quit against Morte if he gets an early Arsenic and I have no Steal. The rest were doable often enough to be worth the try even before I started upgrading my deck). This probably nets you less Electrum per match, but you get in easily twice as many matches since you just abort half of them.
I think this prize discrepancy is ridiculous but that's rooted in the absurd difference in cost between upgraded and unupgraded cards.
I started earlier this week and have maybe 15 upgraded cards now, every one obtained either from false god spins or funded by selling false god spins.
I do think winning upgraded cards from top50 would be too easy. I was farming them with an unupped deck earlier today and ended up making several hundred electrum as well as grabbing almost a full set of Shards (and some weapons). I do think AI5 could use more upgraded cards though so winning them is not such a once-in-a-blue-moon event.