The cost is much lower on a debit card transaction.
Mostly because their fees are even 10 times lower than MasterCard's debit card fees.
The fees for the German Girocard system (which is what colloquially is still called EC) are lower than MasterCard's fees but not by that much anymore nowadays. MasterCard debit should not be 50% more expensive than Girocard considering all fees (which of course still is more) if you shop around a bit. Even Aldi takes them now and they try to save money wherever they can.
Well, Aldi only takes some type of MasterCard debit - only the Chip + PIN edition Maestro cards, not any of the actual MasterCard network, nor mixed credit/debit cards.
Things have changed considerably in the last two years and whatever one knows about cards, acceptance, and fees from before 2015 can safely be forgotten.
But it's so much cheaper than handling credit cards that many small stores in Germany won't accept any cards at all.