The fact of the matter is, winning against a harder opponent nets you more electrum - as it should. This means it should COST more electrum - as it does.
What should be debated isn't the electrum cost/win, it should be the score basis.
But herein lies the problem: if you are just starting out, you might just lose a little (or a lot) against a L0 or L1 (noobs might do this more than you think), so raising the cost should not even be thought of. Think of it this way: when you had (have) a totally unupgraded, non-killer deck, how often did you lose to AI3 until you got the electrum to make a decent deck with decent win %? Your way, you are making this initial grind harder and longer, while also making early score go down logarithmically.
However, if you change the score differential, making losses to early AIs worth more score loss, while FGs are worth less score loss, then you are still making early scores go down way too much.
IMO, it would require way too much work and code to balance this, but an idea would be to have score losses based on your total score, meaning you lose less against AI3 when your score is low than when it is high.
BUT, also take this into account: How often do you REALLY play against AI3 or less? Once you hit FGs and can win 30-50% or so, you are less likely to go back to AI5 unless you get sick of losing or need the electrum. And by this time, you really shouldn't lose against anything but AI5 or T50 anyways.
So in conclusion: Is this really an issue? Do you lose against AI3 enough (or even play them for that matter? I know I don't) for this to matter?