How much is the Word and PowerPoint for MacBook Air???

It's the same price as it is for any Mac. Check the Microsoft Office site.

https://www.office.com/

Apple iWork Pages, Keynote, and Numbers are free from Apple, and can read and write many Office documents.

LibreOffice and OpenOffice are free and open-source packages, and can also read and write many Office documents.

Or go ask Microsoft about their products, pricing, and any offers they might have available.

See: https://products.office.com/en-ca/compare-all-microsoft-office-products?&activetab=tab:primaryr1