Total Profit/Loss = Market Value + Sum of all proceeds (from sells) - Sum of all costs (from purchases)
Market value = Amount of asset held x Current Price
Let's take an example here of Ethereum (ETH). If I have selected GDAX as the source exchange and ETH/BTC as the source pair, BTC is the base pair here. The price per coin i.e. ETH/BTC is taken directly from the exchange. If in USD view mode, it is converted using the same exchange's BTC/USD or BTC/USDT rate to calculate the price per coin in USD. If an exchange doesn't have a BTC/USD(T) pair, then we use the rate from Global Average.
Here is how to change the source exchange and pair.
Net cost = Sum of all costs (from purchases) - Sum of all proceeds (from sells)
and Net cost = 0; if sum of all proceeds > sum of all costs
Therefore, Net Cost can never be negative.
Buy Price / Sell Price:
Price is taken as directly entered by you in the transaction. If the price is in BTC or ETH, it is converted to USD (or your local fiat) using the BTC/USD(T) or ETH/USD(T) from the same exchange if it's available or the BTC/USD or ETH/USD rate from our Global Average for the date and time entered for the transaction.
Worth and Delta:
Worth = Amount bought in that transaction x Current Price
Delta = (Worth - Cost) x 100 / Cost
Avg. Delta = (Avg. Sell Price - Avg. Buy Price) x 100 / Avg. Buy Price
Note: Avg. Delta does not have any significance. It is simply a statistic.