All right, so the electrum reward from beating AI is based on your health left at the end of the fight. It takes the base reward, which is twice the fight cost, then takes a proportion of your health left to it.
If you beat a Level 2 (10 electrum) with 80/100 health left, you get 9. If you have 60 health left, you get 8, and so on.
For Normals and Elders, the lowest you can get is half the base reward if you have 1 health left.
But for Half Bloods, the number can actually go lower than the cost. It's possible to actually lose money even by winning a majority of fights.
Why does it work like this?