What is the price of 256 gb for MacBook Air 13 2017?

You might check Apple's Refurbished list. Refurbished and Clearance.

Apple does not sell SSDs on their own, not do they perform storage upgrades themselves, so no pricing information of that kind is available from them.

You can buy SSDs from third parties however.

Macsales.com is highly recommended for this: https://eshop.macsales.com/shop/ssd/owc/macbook-air/2013-2014-2015-2017

Price is $179.99 for a 240GB drive.