There's a rounding error in the purchase cost formula for units and buildings. It's most visible on faster and slower game speeds.
310
cost for worker on standard
300% cost modifier on marathon
930
target worker cost on marathon
700
actual worker cost on marathon
The cost is 25% lower than the target value. The error is due to rounding multiple times through the cost formula instead of once at the end. If the error was in the ~5% range it wouldn't be a big deal, but 25% off the target is a large enough mistake to significantly alter game balance.