Which would be the way to have different prices for a product depending on what is the packaging selected in the sale order?

What I'm looking for, would be something like this: Some products are packed in 20 unit boxes others on 50 unit boxes others XX unit boxes....

I would like to apply a discount on the product price if it is sold in a box rather than by individual units. I could create a pricelist and apply a diferent prices for a product or product category depending on the quantity but I can't find the way to do it depending on the packaging. As product quaanty per box differ form product to product mantaining it by quantity would make the pricelist to have too many rules.

Thanks for your attention.