Selling by wight at different margins

Selling by wight at different margins

Hi All,

I am trying to figure out how to set up my inventory. I did try enabling Composite items but I still don't understand how to apply the logic as I sell by weight and different margins;

Eg;
I buy a product in bulk, let's say 25 kg. Cost price $2500.
I sell this to customers by weight 250g, 500g, 750g, and 1kg. Each has a different profit margin rate. If the customer buys 250g I sell it with a profit margin of 25% and if they buy 500g it's 20%, so the higher the weight the customer buys margins vary and for 1kg they get a better price, with let's say a margin of 10%. 

How do I add this correctly to the inventory? I created a composite item, and that part is ok, now how do I add products and margins as they vary? And still be able to tally everything?