Cutting LLM Costs Without Cutting Quality: A Practical Playbook

When an LLM bill triples in a month, the instinct is to swap to a cheaper model and accept the quality hit. That is a real lever, but it is the blunt one, and it is usually the fourth or fifth thing worth doing — not the first. Most cost blowouts trace back to waste, not to using an appropriately capable model.

Where the waste actually hides

• Prompt caching left unused — repeated system prompts and retrieved context across calls that a provider will cache for you at a fraction of the cost, if you structure the request to allow it.
• Over-retrieval — pulling twenty chunks into context when the answer only ever needs the top three, paying to process the noise every single call.
• No result caching — identical or near-identical queries recomputed from scratch instead of served from a cache keyed on a normalized version of the question.
• Wrong model for the step — using a frontier model for a classification or extraction sub-task a much smaller, cheaper model handles just as well.

In a recent client audit, restructuring prompts to hit provider-side caching and trimming retrieval from twenty chunks to five cut spend by more than half with no measurable quality change — before anyone touched the model tier.

Route by task, not by product

A single agent often has several distinct sub-tasks — classify intent, retrieve, draft, verify — and they do not all need the same model. Routing cheap, well-defined sub-tasks to a smaller model and reserving the expensive one for the step that actually needs its reasoning is consistently the highest-leverage cost lever we pull, and it is the one closest to free: it is an architecture decision, not a quality trade-off.

Measure cost per successful outcome, not per call

Cost per API call is the wrong unit. Cost per resolved ticket, per correct answer, per completed task is the one that should drive decisions — a cheaper model that needs two retries to get a usable answer is not actually cheaper. Track the outcome-level number before making any model swap, or you will optimize the metric instead of the bill.