Trading mechanics
The spread is the difference between bid and ask; it is one part of the total cost and can vary as market conditions change.
A sell order normally deals from the bid side and a buy order from the ask side. The difference is the spread. Because prices can move between the time an order is submitted and processed, the displayed quote is context for a decision rather than a promise of a particular fill.
Commission, swap or financing, currency conversion, and the way a position is opened or closed may also affect cost. Check the symbol specification and the terms for the relevant account, then include those costs when considering position size and the distance to a stop level.