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.
Enter the price per item in the price field, then select the amout of items you plan to buy at the top just under the image by tapping the plus or minus buttons.
If you go back to your list, you can see that the line reflects the price (per item) and that the total price row at the bottom now reflects the amount x price.