Is ChatGPT Plus Worth $20? Converting Codex Usage into GPT-5.5 API Cost

If you are a ChatGPT Plus user paying US$20 per month, this is worth a closer look, because when you convert that subscription into equivalent API usage, it is much more cost-effective than you might expect.

Starting from actual usage

Suppose you used 5.5M tokens in one month, and that represented only about 7% of your monthly Codex quota.

5.5M tokens ≈ 7% Codex 用量

Working backward, 100% of the Plus plan’s Codex usage would be roughly:

5.5M ÷ 0.07 = 78.57M tokens

How much would 78.57M tokens cost through the GPT-5.5 API?

The current GPT-5.5 API pricing is US$5 / 1M input tokens and US$30 / 1M output tokens. You can estimate the total cost using different input/output ratios.

So when should you switch to the API?

日常開發：繼續用 Plus Codex，非常划算
需要自建 provider routing / key pool：才考慮 API
API 不適合完全取代 Plus Codex，除非你需要產品化、多人使用、或自己的自動化流程

If you have already reached the Codex usage limit, OpenAI also offers additional credits for Plus / Pro users. The Pro plan comes with a larger Codex quota than Plus.

Conclusion

If you are a heavy Codex user, ChatGPT Plus provides far better value than buying the equivalent API usage directly. Unless you need to turn it into a product or share usage across multiple users, Plus is currently the most cost-effective AI coding option.