9 min read • September 2026
Enterprise Sizing Framework
How to Estimate AI Infrastructure Costs for SaaS Applications
Most startup AI budgets fail because they multiply average prompt tokens by active users and stop there. Real-world bills are driven by peak concurrency overprovisioning, failure retries, compounding agent turns, and vector database IOPS.
1. The 5 Variables Every AI Budget Must Include
1. User Activity Volume (DAU × Requests/User)
Total monthly interactions = Daily Active Users × Average queries per day × 30.
2. Token Asymmetry Ratio (Input vs Output)
Separate input tokens (system + RAG + history) from output tokens (model response + reasoning chain).
3. Prompt Cache Hit Percentage (0% to 90%)
Estimate how much of your input context is static and reused across sessions.
4. Retry & Guardrail Penalty Buffer (10% to 25%)
Factor in schema parsing retries, moderation guardrails, and timeouts.
5. Vector Storage & Neural Reranker TCO
Include vector DB monthly namespace charges and reranking fees (e.g. Cohere Rerank $1.00/1k).
2. The Complete Monthly Forecasting Formula
Total Monthly Spend =
Monthly Queries × [ (Uncached Input × Input Rate) + (Cached Input × Cache Rate) + (Output × Output Rate) ] × (1 + Retry Rate) + Auxiliary Costs (Embeddings + Vector DB + Tools)
Simulate Your SaaS Unit Economics
Use the Monthly AI Cost Calculator for customer margin planning.