I payed 7 Euro transfer fee from bitcoin.de to my Portis wallet.
Transfer fees from Portis to Stakewise are 38 Euro.
Why is the latter so much more expensive?
Every interaction with the etherium blockchain costs a quantity of gas. The user has to provide that quantity times the prevailing gas price in ether to complete the transaction. For a simple transfer of ether the gas required is 21000. However when depositing ether into Stakewise you are interacting with a smart contract and the gas required is approximately 4 times as much. This is actually quite low compared to some smart contracts. Any remaining difference is likely to be the difference in gas price at the time you made the two transactions.