MacBook Pro RAM upgrade pricing is now exactly the same as MacBook Air RAM upgrade pricing. So, my very strong guess is that it was somehow “discounted” at launch and has now returned to “normal”.
Nothing to do with component cost/availability, or anything else.
(Apple should honor previous orders at the previous pricing.)
https://www.macrumors.com/2020/05/30/13-inch-macbook-pro-ram-upgrade-doubled/