Hi AdamSmith,

There is no fee using Locks.  For transactions up to 50 BTC we keep the bid-ask spread between 2 and 4%. This means each transaction will be 1-2% away from our index price, which closely mirrors prices at the major exchanges. Most of the time the spread will be near the minimum, only venturing near the maximum when the market becomes abnormally illiquid.

Unlike exchanges, Coinapult guarantees the price quoted for 15 minutes.  The point is, you'll know the price before you act.