ChatGPT Plus is listed at $20 monthly, but the actual price you pay can vary...

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

ChatGPT Plus is listed at $20 monthly, but the actual price you pay can vary between $13 and $27. Differences come from taxes like VAT or GST and extra fees from app stores on iOS and Android