In most EU countries there is a law that requires anyone selling any products that can be measured with a physical quantity, like meters, kilograms, grams, liters, sheets, pieces, pairs, etc. to show the price per physical unit together with the product's price.
Example: I am selling 200ml bottles 'Water from the Fountain of Youth' for 50 Euros. Then I must show the pricing for 1 liter also: 250 EUR/liter. It is also regulated that this information cannot be written in a too small font or hidden into some description text, it must have a comparable readability as the main price label. (own experience: operating a 1800 item webshop and forgetting to show the price per unit for only a few items could cost you 1800 EUR) It is a serious issue, I think any webshop engine should help to avoid.
Currently all shops (especially in Germany where they fear the most of the consequences) manually calculate these numbers and write these markings into some description field like the short description of a product. Or the other option they hack the 'old price' field for this. It is a nightmare (unfeasible) though with discounts or even with simple price change to update this value each time.
I suggest to introduce a product category flag and an extra price field that
1. would not allow this to be mistakenly left out from a product
2. would show the 'price per unit' with a similar visibility as the product's main price in all product listing where a price is visible
3. it would be perfect only if the (re)calculation would be automatic when a product's price changes