I think you misunderstood my previous post about the fees.
The withdrawal fee you pay is always the fee a 1 kB transaction would need (hence "you pay the per kB fee").
If your transaction is bigger than 1 kB (eg because there are several inputs used, which quickly amount to a huge transactions, as they are way bigger than outputs), then the site takes up the remainder of the fees.
If your transaction is smaller than 1 kB, then you end up paying more fees than actually used.
As users can't influence how big their withdrawal transactions are, and it's hard for the site to predict how big those transactions will be, before actually sending it,
all users are paying the same fee (as in, the fee for 1kB), regardless of transaction size. This way, the system is fair for everyone.
therefore I paid a significantly larger fee than I thought I would.
The withdrawal window tells you exactly how big a fee you pay and how much you receive from your total withdrawal amount.
I just don't know why the fee system isn't clearer and also cheaper.
I think it is clear, the only thing not mentioned on the withdrawal page itself is that the fee is per kB (which is mentioned in the FAQ, though).
You can see that a 0.1
BTC withdrawal with a 0.00108
BTC fee gives you a value of 0.09892
BTC sent to your wallet (0.1-0.00108=0.09892).
As for the costs, that's mainly related to Bitcoin transaction fees, fees for eg litecoin are fairly low if you want alternatives.