How can I reference a field from a product display when calculating a price?
I generally recommend any information required for the pricing / fulfillment of a product to be stored on the product itself. I can see where you might need to do so based on information in the display, though (e.g. discounting products in a particular category).

In that case, you can't traverse from the line item / product to its display mode via the standard Rules data selector. Instead you need to use the appropriate action to load the node that references the product in question. Then you'll need to pass the line item and that node to a Rules component where you can use additional conditions to bring field data from the display node into scope (e.g. "Entity has field") and use the line item unit price manipulation actions on the line item to affect the pricing.