First of all, you need to turn on price crowdsourcing under settings (only done once).
Tap the shopping list item in question, e.g. apples.
Then tap the name field to expand additional detail fields and enter the weight and unit you are buying.
Set the unit to the unit that the price is given for, e.g. if price is per kg then enter how many kg you buy.
A toggle will appear just above the note field that lets you toggle ON price per unit.
If you go back to your list, you can see that the line reflects the price (per unit) and that the total price row at the bottom of the list now reflects the amount x weight x price