Progressively increasing weight and items?

I have set progressively increasing weight and according prices. My problem is that I want a different extra cost for every case adding an item in the cart.

I have made it working for a fixed price using the examples

item: min 1 – cost: -15
item: min 1 – cost: 15 – additional cost: 15 – value: 1

The problem is here

weight: min:1 – max: 2 – cost 5
weight: min:2.1 – max 3 – cost 6
etc…………
weight: min:9 – cost 12.

I want per item to find the weight and add the cost of the item if I add 2 items I want to take the weight of the item and double it if he adds 3 items triple it etc.

and If I use different products it adds the weight I want only per item the cost

example If a customer adds an object weights 1.5kg cost 5
and an object 2.5kg cost 6
I want to summarise the weight per item and not to summarise the weight

total cost 11

If the orders 2 items with weight 1.5kg cost 5×2 = 10
2 items weight 2.5kg cost 6×2 = 12
total cost 10+12=22