Some numbers need a calculation on each row before you add them up. Revenue before discount is quantity times price on every order line, then summed. DAX gives you two ways.
A calculated column adds a column to a table and fills it row by row. In Table view, select a table and choose New column:
Line Profit = Sales[Sales Amount] - Sales[Cost]
An iterator such as SUMX does the row-by-row work inside a measure and stores nothing:
Gross Sales = SUMX(Sales, Sales[Quantity] * Sales[Unit Price])
Don't write SUM(Sales[Quantity]) * SUM(Sales[Unit Price]): that multiplies two totals, which is a different and much bigger number.
While SUMX is on a row of Sales, RELATED reaches across the relationship to the matching row of another table: RELATED(Product[List Price]) is the list price of that row's product.