How do I calculate discounts for pricing plans for bigger volumes of a product?

We have three pricing plans in b2b SAAS. We charge our customers price = cost (cloud provider) + overhead (our service). let's say our overhead is 20% for cheapest plan. We want customers to buy or upgrade to more expensive plans by making our overhead lower (e.g. 15% overhead). Is there a good financial / optimisation approach which would allow us to find best overheads based on volume of things customer buy? So we could optimise simultaneously price for customer (lowering price per unit) and our profit? Sorry if it's super naive question, but I'm not even sure where to start googling