Calculated prices per item and not order

[dlhck]

Migrated from #1365

Is your feature request related to a problem? Please describe.
The prices won't be displayed to the customers before they add it to the cart.

Describe the solution you'd like
I will like to show them if a certain product is on sale, or if eg. a product is cheaper if the customer would bulk buy. Like eg:
Item A is on sale for 20%. 1 for 20. Or buy 5 for 15/piece
This would lead to better sell.
Technically it would be nice to have salesPrice in the item. Maybe with configuration where you can customize the calculation of the price.
There will in my case be:
Discount for an item as % or fixed price.
Discount for a customer group as % or fixed price.
Discount for a customer within price group as fixed price.
Discount for a customer within discount group as %.

Calculation in short:

1. Take the lowest possible fixed price.
2. Take the lowest possible % discount.
3. Add % discount to the fixed price.

Describe alternatives you've considered
I've disabled the default price field on productVariant and added custom price which includes default price and salesPrice.
In the salesPrice, it would pull ALL promotions, and then filter them, and then calculate. Imagine doing this for every products in a queried list of products. This slows down the query pretty much. It is also a problem for ordering, as the price is in a custom field.