Before building an AI-powered feature, you need a realistic cost estimate. Underestimate and you blow your budget. Overestimate and you might kill a project that would have been profitable. Here's how to get it right.
Step 1: Understand Your Usage Pattern
Every AI API cost comes down to three variables:
- Tokens per request — How much text goes in (prompt + context) and comes out (response).
- Requests per day — How often your users trigger the AI.
- Model choice — Flagship models (GPT-4o, Claude Opus) cost 10-50x more than budget models (GPT-4o mini, Haiku).
Step 2: Estimate Token Counts
A rough rule of thumb: 1 token ≈ 4 characters in English, or about 3/4 of a word. Here are typical token counts for common use cases:
| Use Case | Input Tokens | Output Tokens |
|---|---|---|
| Chatbot reply | 500 | 300 |
| Document summary (5 pages) | 4,000 | 500 |
| Code generation | 2,000 | 1,000 |
| RAG query | 8,000 | 500 |
| Long report analysis | 40,000 | 2,000 |
Step 3: Pick the Right Model Tier
Don't default to the most capable model. Match the model to the task:
- Simple classification/extraction: GPT-4o mini, Claude Haiku, Gemini Flash ($0.10-0.15/M input tokens)
- General-purpose tasks: GPT-4o, Claude Sonnet, Gemini Pro ($1.25-3.00/M input tokens)
- Complex reasoning: Claude Opus, o3, GPT-4.1 ($2.00-15.00/M input tokens)
Step 4: Calculate Monthly Cost
The formula is straightforward:
Monthly Cost = (Input Tokens × Input Price + Output Tokens × Output Price) × Requests/Day × 30
For example, a chatbot using GPT-4o mini (500 input, 300 output tokens per request, 1,000 requests/day):
(500 × $0.15/M + 300 × $0.60/M) × 1,000 × 30 = $8.10/month
The same chatbot on GPT-4o would cost $105/month — 13x more. Model choice is the biggest lever.
Step 5: Plan for Growth
Multiply your estimate by 2-3x for a realistic budget. Usage patterns change, prompts get longer with added features, and successful products attract more users.
Cost Optimization Strategies
- Start with the cheapest model that works — Test your use case on budget models first. Upgrade only when quality demands it.
- Use prompt caching — Anthropic and OpenAI offer cached prompt pricing at 75-90% discount for repeated system prompts.
- Batch non-urgent requests — Batch API pricing is typically 50% off standard rates.
- Shorten your prompts — Every token costs money. Tighten system prompts, remove unnecessary context.
Use our AI Project Cost Estimator to get a detailed breakdown for your specific project type, or the AI Token Cost Calculator to compare individual model pricing.