Generally you'd just want it so the better (more expensive) units end up being more cost-effective.
Basically all building stronger units does is mean you have to do less clicking, but ultimately you'll do worse overall.
The best unit costs 275 gold, the worst costs 8.
So we do 275 divided by 8, which is roughly 34. Meaning we can build 34 of the worst unit, or 1 of the best unit.
34 of the worst unit will have 340 HP and 340 Attack.
1 of the best unit will have 250HP and 250 Attack.
Obviously in this case, it's better to just spam those 34 units, and never touch the most expensive unit, as it's a waste of money.
It's a fine act of balancing to get a challenging and fair game, I think you have that, but it just gets skewed a little with the 2nd and 3rd units. The Player-Enemy power is fairly well made.