How to set product price with rules on checkout
You must be using the product pricing system, which requires a rule assigned to the "Calculating the sell price of a product" event. Rules on this event can make use of a variety of Line item related actions to manipulate the unit price of the line item, and you'd just add conditions to your rule to only apply pricing based on the product SKU.

See the FAQ on Rules data selection for information on checking the product SKU:

https://drupalcommerce.org/faq/rules-field-data-selection