AI Cost Economics

Token pricing 101

LLM API tính tiền theo input + output token. 1 token ≈ 4 chars English, ≈ 2.5 chars Vietnamese (vì diacritic).

Ví dụ pricing (Q1 2026)

Unit economics PM phải biết

• Cost per call — phụ thuộc prompt length + completion length.
• Cost per user/month — # calls × cost/call.
• Margin — ARPU − AI cost − infra − support.

Sample calculation

App chatbot: user trung bình 20 message/tháng, mỗi message 2k input + 500 output token, dùng GPT-4o. Cost = 20 × (2k × $2.5/M + 500 × $10/M) = 20 × ($0.005 + $0.005) = $0.20/user/month. Nếu sub là $5/month, margin AI ~96%.

Optimization techniques

• Prompt caching — cache system prompt + few-shot, OpenAI/Anthropic discount 50–90%.
• Model routing — small model cho easy query, big model cho complex.
• Output capping — max_tokens limit để tránh runaway.
• Batch API — 50% discount nếu non-realtime.
• Embeddings cache — reuse embedding cho user query repeat.
• Compression — shorten prompt bằng abbreviation + structure.
• Fine-tune — task narrow, volume cao có thể profitable.

Vietnam context

VND/USD exchange + VAT làm consumer AI feature VN khó margin tốt. Nhiều startup VN dùng VinAI PhoGPT hoặc open-source local (Llama 3, Qwen) để tránh token cost USD.

Common pitfalls

• Không monitor cost daily → bill spike sau viral moment.
• Default max_tokens too high → 90% call dùng <500 token nhưng cap 4k.
• Skip caching analysis → leave 50% saving.
• Over-engineer fine-tune cho task <1M call/tháng.