What is Difference between what the store pays for an item and what the customer pays?

Mark up.

Here is a sample sentence.

"These people are making a lot of money. They buy the product for a few cents but the mark up is a few dollars."

"The mark up on this items is ridiculously large."

It comes from the verb, to "mark up."

Clearly, the process of writing the price on an objected can be understood as marking the price. If the price you mark is higher, on can see how the term "mark up" would have been created.