The OpenAI API Cost Calculator helps users estimate the costs associated with using GPT-3.5 and GPT-4 via API. It allows individuals to determine whether subscribing to ChatGPT Plus or utilizing the API is more cost-effective based on their usage patterns. The tool provides insights into pricing based on prompt length and token usage, enabling users to access advanced features and settings not available in the standard ChatGPT interface. By inputting their usage frequency and interaction types, users can make informed decisions about their AI service options.