I really dislike anything below 100. I would say 300-500.
Of course, you want to grade it based upon the mode. Not sure what smaller accounts look like these days. Should keep energies lower for them to keep normal mode "entry level". Prestige begin at at least 100 (would rather see 200). Master begin at 300
Don't have to worry as much about smaller players when tweeking master mode. Most who go for it have higher energy pools & rates to draw on. For master, the higher the better... goal is the least amount of taps possible.
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may also want to consider lowering the total energy again. The boosts are not good anymore and there are no special boosts offered. Raising the total energy doesn't make sense. If you lowered it back to 20k/day for prestige & 60k/day for master, I'll bet you'd see an increase in those attempting it.
Orrrrrr try this -
Release an LTQ with a special boost (Saaaaay an infantry unit called The Danimal with a +3 Energy regen boost )
Keep the total energy within reason and I bet both participation and gold sales increase... a special boost hasn't been offered in awhile - the community would go nuts for it