So having tried to test it myself but not getting a simple answer, then making my way through the old Game Coding Q&A topic and not finding one there either:
What's the formula for determining Electrum/Score Gain for HP values > 100?, i.e Plat was worth 222 today, I tested a 124HP EM and won 496 electrum, a 172HP EM and won 602 electrum. I also beat the rank 4 deck with a 148HP EM for 1,100 which I believe doubles the value, assuming that's correct then electrum gain for EMs:
100HP = 444
124HP = 496
148HP = 550
172HP = 602
The scaling is linear at + 52 per extra 24HP for 100 to 124 and 148-172, but not for 124-148 where it is + 54. That also gives a ratio of 13 electrum : 6HP which seems a little odd, although still plausible.