Comparing Frontier Model API Prices: Grok 4.5, GPT 5.6, Opus 4.8, Fable 5

Melvin Vivas · X post · 2026-07-09 · Open on X

Topics: LLM Fundamentals, LLMOps, Deployment & Monitoring · Level: beginner

Summary

Lists API prices per million tokens for four frontier models. Use it to compare cost when you choose a model, and notice that output tokens cost much more than input tokens.

Key points

• Grok 4.5: $2 input / $6 output per million tokens (the cheapest of the four)
• GPT 5.6: $5 input / $30 output
• Claude Opus 4.8: $5 input / $25 output
• Fable 5: $10 input / $50 output (the most expensive)
• Output tokens cost 3-6x more than input tokens, so output-heavy workloads such as agents and code generation change the cost a lot

Resources mentioned

• Grok 4.5 · tool · x.ai · paid
  An LLM said to be trained in partnership with SpaceXAI, pitched as a general model beyond software engineering.
  Also in: Grok 4.5 Works Well as the Model Behind Hermes Agent (Melvin Vivas on X · notes), Models That Work With the Hermes Agent for Personal Productivity (Melvin Vivas on X · notes), Agent-Made Video in 10 Minutes: Hermes Agent + Grok 4.5 + Hyperframes (Melvin Vivas on X · notes), Personal Assistant Agent on Hermes: Morning Briefings and Inbox Triage (Melvin Vivas on X · notes) and 9 more
• GPT 5.6 Luna · tool · openai.com · paid
  The OpenAI model used inside Codex for the demo. The transcript gives the variant name as 'Soul', which is unclear.
  Also in: Set Codex subagent model and reasoning to save usage limits (Melvin Vivas on X · notes), Use GPT-5.6 Luna in Codex for Terminal Tasks (Melvin Vivas on X · notes), Match Reasoning Effort to Task Length in Codex (Astra/Sol) (Melvin Vivas on X · notes), Run Coworker desktop agents cheaply with GPT-5.6 Luna on OpenRouter (Melvin Vivas on X · notes) and 24 more
• Claude Opus 5.5 · tool · claude.ai · paid · open in a browser to verify · recommended by both Bashiri Smith & Melvin Vivas
  Anthropic's AI assistant, used throughout the guide to tailor resumes, add live roles to the tracker, match connections to target companies and find hiring managers.
  Also in: Generating a Repo Promo Video with a Claude Skill on Sonnet 5.5 vs Opus 5.5 (Melvin Vivas on X · notes), Comparing Coding Models on the Same Task in Devin iOS (Melvin Vivas on X · notes), Customizing Your Coding Setup with Pi Coding Agent Extensions (Melvin Vivas on X · notes), AI Engineer Roadmap Overview: From ML Foundations to RAG, Agents & Ops (Bashiri Smith on Facebook · notes) and 53 more
• Fable 5 · tool · anthropic.com · paid
  A frontier model used as the comparison point in the Claude Opus 5 announcement; the post gives no other details about it.
  Also in: Claude Opus 5.5 in Devin: #1 on FrontierCode 1.1 (Melvin Vivas on X · notes), Creator's Top 3 Closed Models: Fable 5, GPT 5.6 Sol, Grok 4.6 (Melvin Vivas on X · notes), Opus 5 vs Fable 5: Cost per Task in Cursor Benchmarks (Melvin Vivas on X · notes), Opus 5 Beats Fable 5 on Cost per Task and Tokens (Melvin Vivas on X · notes) and 7 more

Try this

• Compare input and output token prices against your workload before you choose a model