It makes it a way too good upgrade, since you can quickly buy all the cheap upgrades and the next "tier" costs a lot more anyways simply because of this upgrade.
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There's some balancing to be done, but that particular upgrade boosting income dramatically is kind of the point from what I can tell. It's a gateway ability designed to move the player into the next income bracket, thus letting them explore the next tier of upgrades. In other words, it's a feature, not a bug. XD I've seen it in other incremental games.