Well, it helps a lot to know what kind of party you want and strive for it throughout the entire game. It is also preferable to stick with the army you like and refrain from purchasing additional troops, simply because you found better ones on the next island. But rest assured
Once you make the perfect army, you won't feel the need to change your troops constantly, and the money will pile up.
I can confirm that KBAP is indeed tougher money-wise. For referrence:
KBTL Hard VS KBAP Hard
Enemy strength: 130% vs. 130%
Gold income: 77% vs 75%
Rage growth: 85% vs 85%
Spirits/Dragon gorwth: 85% vs 85%
KBTL Impossible vs KBAP Impossible
Enemy strength: 170% vs. 170%
Gold income: 60% vs 50%
Rage growth: 70% vs 70%
Spirits/Dragon gorwth: 70% vs 70%
As you can see, not only the trading skill is gone, but on the higher difficulties, the money income has also been reduced.