How to implement bulk pricing/discounts with Drupal Commerce?
Maybe you should use the Commerce Discount module. Quote from its community documentation:


  The Discount UI (available under Admin → Commerce → Store → Discounts) allows creating and editing discounts. Each discount consists of two related entities: Commerce Discount and Commerce Discount Offer. The Discount allows you to choose what type of entity this discount this will apply to i.e. "Order Discount" or "Product Discount" (line item). The Commerce Discount Offer denotes the result the discount produces. The module provides 4 discount offers out of the box.