Why does the calculation of TBanque positions include both BUY and SELL prices?

Positions on TBanque are calculated to cover the entire transaction, at the time that the position is opened. 

This means that you get the full picture of the cost up front. 

When you close the position, the closing fee is adjusted to reflect the market price of the cryptoasset at that time, so the spread cost shown is subject to any movements in the market price that may have occurred between the time that you open and close the position.

Note: This model does not apply to stocks and ETFs which are labelled as “zero commission” in which no additional fees or mark-ups are incorporated within the price. For TBanque fees for stocks and ETFs, see here.