One feature most people probably don't notice in Numbers is that when you have a Header Row then each column automatically becomes a named range. Thus in the formula editor you can easily enter formulas using names rather than cell addresses.
Here, I started typing Pr and used the right arrow to navigate over to Price and hit enter. The formula editor inserted the appropriate reference.

Then I typed * for multiplication followed by Qu and right arrow to Quantity and hit enter.

Then I filled the formula down.
When you look in one of the cells in column D you will see this;

If at Numbers > Preferences you turn on "Use header names as labels" you will see this.

Under the hood the named ranges are always active and you can often take advantage of them when entering formulas and debugging formulas.
SG