Monthly Budget Estimation: Build a 30-Day Forecast in 5 Minutes

The Five-Minute Forecast

To estimate monthly cost before launch:

1. Pick the model you're 90% sure you'll use in production.
2. Estimate per-call input + output token counts based on a realistic prompt example.
3. Estimate daily call volume: DAU × queries/user/day. For B2B SaaS, 100-500 calls/user/day is common; for consumer chatbots, 5-20 is typical.
4. Multiply by 30 for monthly volume.
5. Add 25% safety margin for spikes, retries, and underestimation.

Example: Internal customer support bot

• Model: GPT-4o
• Per-call: 8,000 input tokens (system prompt + RAG context + question), 800 output tokens (answer)
• Daily volume: 200 employees × 15 queries/day = 3,000 calls/day
• Monthly: 90,000 calls

Per-call cost: 8K × $2.50/1M + 800 × $10/1M = $0.028 Monthly cost: 90,000 × $0.028 = $2,520 With 25% margin: $3,150

Example: Public-facing chatbot

• Model: GPT-4o mini for cost (with Claude Sonnet for hard queries)
• Per-call: 3,000 input, 500 output
• Daily volume: 50,000 DAU × 8 queries = 400,000 calls/day
• Monthly: 12,000,000 calls

Per-call (GPT-4o mini): 3K × $0.15/1M + 500 × $0.60/1M = $0.000750 Monthly: 12M × $0.00075 = $9,000/month

Example: Code agent for 100 engineers

• Model: Claude Opus 4.7 with caching
• Per-PR: 50K input tokens × 10 turns = 500K input, 10K output
  • Without cache: 500K × $15/1M + 10K × $75/1M = $8.25/PR
  • With cache (80% hit rate): 500K × (0.2 × $15 + 0.8 × $1.50)/1M + 10K × $75/1M = $2.85/PR
• Volume: 100 engineers × 5 PRs/day × 22 working days = 11,000 PRs/month
• Monthly (cached): 11,000 × $2.85 = $31,350

Common forecast mistakes

• Forgetting retries: 5-10% of API calls fail and get retried. Add 7% to your call count.
• Ignoring system prompt growth: prompts grow 2-3x over a product's first year.
• Underestimating output: users hit "regenerate" and "tell me more" — assume 1.3x the planned output length.
• Skipping margin: launches surprise everyone. Always +25%.

When the forecast is wrong

Set a Datadog/Grafana alert at 70% of monthly budget on day 14. If you're tracking above 70% on day 14, you're on pace to overspend by 50% — investigate immediately. The most common culprit is a runaway loop or a misconfigured retry policy.