Ok so... words words words.
So whats new you ask?.... Well ive been day trading ETH on Bitstamp last 2 days.. Managed to make a 100 Euros. Woohoo. Waiting to get a good deal on Steem, maybe if it drops by a bit in relation to ETH id switch it. Really hoping ETH could do a XRP type of run soon so i can stock up on STEEM and HODL till the SMTs come in.
Trying to figure out 
The closest way i found to see how much they charge you is by going on a website like the one i found with the same name as blocktrades.us (this blocktrades isnt related to blocktrades.us) and opening the "crypto-converter"... Heres the link: https://blocktrades.org/cryptocurrency-converter
Typing in the currency you will be using alongside STEEM and then compare that with the estimated amount on the blocktrades exchange.
For me it came out to be around 2.6%
But even that most likely isnt correct because it seems that converters like that one, or the one on "Wallet investor" or "Currencio" or "coinmill" show vastly different results. In the end you have absolutely no idea where you stand...
Therefor itd be great if blocktrades.us would give us some idea, during purchase of how much they pocket. :)