ChatGPT Plus is listed at $20, but the actual price can range from $13 to $27...

https://www.slideserve.com/aaron_ward92/why-does-chatgpt-plus-cost-more-on-android-than-on-the-web

ChatGPT Plus is listed at $20, but the actual price can range from $13 to $27 depending on where and how you subscribe. Differences come from taxes like VAT or GST and extra charges from app stores