Blocktrades also charges a fee which seems to be larger on small amounts. For example, here are the numbers right now I see.
10 SBD = 0.00137956 BTC
100 SBD = 0.02043332 BTC
1000 SBD = 0.20694816 BTC
Therefore the best rate is given currently out of those three when sending 1000 SBD with the payout for 10 SBD to 100 SBD which is 10 times more actually coming out to 14 times more. For this reason, I have asked Blocktrades to please be transparent about the exact exchange rate and the fee taken to help us understand how to best make our exchanges. Send at least 100 SBD at once to get the best rate!