No, the game says it's 1 runik = 2.67 wheat. Then it says there is a 70% tax. So it should charge 4.539 wheat after tax but instead it tries to charge 10.6 wheat for 1 runik...

That's a 400% tax.

The math probably tries to multiply something by the number of non selected resources would be my guess. Either way, it's wrong and should be acknowledged and fixed by either correcting the tax rate shown or correcting the formula if you think the current numbers are in a good spot.