Table of Contents
For finance teams evaluating self-hosted AI vs hosted-API alternatives, ROI math goes beyond per-token comparisons. The real picture includes direct cost saving, productivity gains, residency value, and the engineering ops cost that hosted alternatives include in price.
Three components: direct cost saving (typically 60-80% vs frontier API at scale), productivity gains (often the larger figure for internal tools), compliance / residency value (hard to quantify but real for regulated industries). Hidden costs: ~£500-1,500/mo of engineering ops, monitoring, eval maintenance.
Framework
For a 12-month ROI calculation, capture:
- Saved API spend: previous frontier-API cost − new hybrid (self-hosted bulk + API fallback) cost
- Productivity gains: time saved across users (internal tools); customer satisfaction lift (external products)
- Compliance value: avoided audit overhead, simplified procurement
- Engineering ops cost: ~0.5-1 FTE pro-rated for self-hosted maintenance
- Server cost: dedicated GPU monthly
Metrics
Concrete metrics for ongoing tracking:
- £/M tokens served: tracks model + hardware efficiency
- Cache hit rate: prefix + semantic; correlates directly with cost
- p99 latency: user experience proxy
- Eval score over time: quality drift detection
- Hosted-API fallback rate: how often hardest cases need frontier model
Hidden costs
- Engineering ops time: ~5-10 hours/week ongoing for monitoring, model updates, tuning
- Eval harness maintenance: ~5 hours/week to keep eval representative
- Model drift response: occasional re-tune when model behaviour changes
- Scaling decisions: when to add replica, when to upgrade tier
- Security patching: OS + dependencies + driver updates
Verdict
For honest ROI on self-hosted AI: account for ops engineering time, not just server cost. The total of self-hosted (server + ops + fallback API) is still typically 40-70% cheaper than pure frontier API at production scale. Compliance / residency value adds material upside for regulated industries.
Bottom line
Account for ops time; net saving still strong. See unit economics.
