Next upgrade at [threshold] saved [Increase interest [price]]
Threshold should increase 10-logarithmically, so the first upgrade can be bought at $1 saved, the next at $10 saved, the next at $100 saved, and so on.
Price should increase 2-logarithmically, so the first upgrade can be bought for $1, the next for $2, the next for $4, and so on.
Mockup:
Threshold should increase 10-logarithmically, so the first upgrade can be bought at $1 saved, the next at $10 saved, the next at $100 saved, and so on. Price should increase 2-logarithmically, so the first upgrade can be bought for $1, the next for $2, the next for $4, and so on.