At $10/month, ChatGPT-4 is reasonable. At $20, that's a no no!
The other effect at work here is anchoring. OpenAI wants people to be prepared to pay more for AI. Setting a higher price anchors that price in people’s minds and makes it feel normal. This will serve them better in the future since the long-term objective is to provide services that replace employees, who are surely paid more than $20/month.