Writing test class for PricebookEntry

@isTest
public class PriceBookTest {
    // Utility method that can be called by Apex tests to create price book entries.
    static testmethod void addPricebookEntries() {
        // First, set up test price book entries.
        // Insert a test product.
        Product2 prod = new Product2(Name = 'Laptop X200',
            Family = 'Hardware');
        insert prod;
       
        // Get standard price book ID.
        // This is available irrespective of the state of SeeAllData.
        Id pricebookId = Test.getStandardPricebookId();
       
        // 1. Insert a price book entry for the standard price book.
        // Standard price book entries require the standard price book ID we got earlier.
        PricebookEntry standardPrice = new PricebookEntry(
            Pricebook2Id = pricebookId, Product2Id = prod.Id,
            UnitPrice = 10000, IsActive = true);
        insert standardPrice;
       
        // Create a custom price book
        Pricebook2 customPB = new Pricebook2(Name='Custom Pricebook', isActive=true);
        insert customPB;
       
        // 2. Insert a price book entry with a custom price.
        PricebookEntry customPrice = new PricebookEntry(
            Pricebook2Id = customPB.Id, Product2Id = prod.Id,
            UnitPrice = 12000, IsActive = true);
        insert customPrice;
       
        // Next, perform some tests with your test price book entries.
    }
}

Source