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How to Estimate AI API Costs Before You Build
Published May 3, 2026
Before committing to an AI model for your product, estimating the monthly cost saves you from surprises. This guide walks through the calculation step by step.
The Core Formula
All major AI API providers charge separately for input and output tokens:
Cost per request = (input_tokens / 1,000,000) × input_price
+ (output_tokens / 1,000,000) × output_price
Then scale to daily and monthly:
Monthly cost = cost_per_request × requests_per_day × 30
Step 1: Measure Your Token Counts
Before estimating, know how many tokens your typical request uses.
Input tokens include:
- Your system prompt (often 100–500 tokens for a well-written prompt)
- Any conversation history you pass with each request
- The user's message
Output tokens are harder to predict — they depend on what the model generates. Run 10–20 representative queries and measure the average response length.
A rough guide for response types:
| Response type | Approximate output tokens |
|---|---|
| Short yes/no answer | 10–50 |
| Paragraph summary | 100–200 |
| Detailed explanation | 300–600 |
| Long-form content | 800–2,000+ |
Step 2: Choose Your Model
The cost difference between models is large. For the same workload, here is what a 1,000-token request (500 in, 500 out) costs across models:
| Model | Input price (per 1M) | Output price (per 1M) | Cost per request |
|---|---|---|---|
| GPT-4o mini | $0.15 | $0.60 | $0.000375 |
| Claude 3.5 Haiku | $0.80 | $4.00 | $0.002400 |
| GPT-4o | $2.50 | $10.00 | $0.006250 |
| Claude Sonnet 4 | $3.00 | $15.00 | $0.009000 |
For high-volume workloads (1,000+ requests/day), the cheapest tier can be 20–40× less expensive than the premium tier.
Step 3: Estimate Request Volume
Map your use case to daily requests:
- Internal tool (10 users, 20 req/user/day) → 200 req/day
- Startup chatbot (100 daily active users, 5 conversations each) → 500 req/day
- B2B SaaS feature (1,000 daily active users, 2 req each) → 2,000 req/day
Step 4: Run the Numbers
Example: Chatbot with GPT-4o mini, 500 input tokens, 300 output tokens, 1,000 requests per day
Cost per request:
Input: (500 / 1,000,000) × $0.15 = $0.000075
Output: (300 / 1,000,000) × $0.60 = $0.000180
Total: $0.000255
Daily cost: $0.000255 × 1,000 = $0.255
Monthly cost: $0.255 × 30 = $7.65
A $7.65 monthly bill for 30,000 requests is typical for small-scale chatbots on budget models.
Common Cost Traps
Long system prompts — A 1,000-token system prompt attached to every request adds 1,000 input tokens per call. At 10,000 requests per day, that is 10 million extra tokens monthly.
Conversation history — Passing the full chat history grows the input token count linearly with conversation length. Truncate or summarize older turns.
Re-processing the same context — Some providers offer context caching at a discount for frequently repeated content (e.g., a large document you reference in every request).
Choosing the Right Model Tier
Start with the cheapest model that meets your quality bar. Reserve premium models for:
- Complex reasoning tasks
- Cases where output quality directly affects revenue
- Low-volume, high-stakes decisions (legal review, medical summaries)
For most classification, summarization, and Q&A tasks, GPT-4o mini or Claude 3.5 Haiku deliver quality close to their premium counterparts at a fraction of the cost.