Are devils worth 7 blood each?

Returning to the main topic of the thread, I thought I'd do another ROI calculation for purposes of comparison.

A typical Arco early expansion army might consist of 1 commander, 1 priestess, 16 Hypaspists and 4 Elephants. Total cost approx 800 Gold. Assume this army conquers 5 provinces with an average value of 40 Gold each, before attrition or bad luck ends its progress.

So, total income from the conquered provinces is effectively 146 gold (we lose about 54 per turn to pay maintenance on the army), which means our investment in elephants pays back in a little under six turns. That's pretty damn good; an awful lot better than you could expect in real life.

I wonder if it's unbalanced?

Mark