The price for the migration is calculated based on the Entity Limit.
Entity Limit is calculated using the highest entity number of products, customers and orders. If your Entity limit is 1000, that means you can migrate 1000 customers, 1000 orders, 1000 products. Besides, you can move all categories and all other supported data types. Accordingly, if you want to migrate 1000 products, the Entity Limit will be 1000 and the price will remain unchanged.