Two voice AI developers using the same platform (like Retell AI), can build agents that sound identical, behave the same way and show no noticeable difference in latency, yet one agent can cost significantly more to operate than the other.
The difference is often hidden in how each developer configures the underlying system. For example, one may use an expensive language model when a cheaper model performs equally well.
They may select a premium voice, enable unnecessary add-ons, send every call through paid quality assurance, or buy additional concurrency the client doesn't need.
Call design also affects cost. A more efficient agent can achieve the same outcome with shorter calls and earlier transfers.
Building a good voice agent is not just about how it sounds or behaves. It must also be cost-effective to operate.
In this article, I share 15 proven strategies to build cheap Retell Voice Agents.
But before we get to the strategies, we need to understand how Retell charges.
Retell uses usage-based pricing and offers two plans: ‘pay-as-you-go’ and ‘Enterprise’.
There is no base platform fee on the pay-as-you-go plan.
Your bill is assembled from the services used during each connected call:
The practical formula is:
Voice cost = billed minutes × (infrastructure+LLM+voice+telephony+add-ons)
Then add any:
- Per-call charges.
- Per-dial charges.
- SMS charges.
- Phone-number subscriptions.
- Additional concurrency.
- Additional knowledge bases.
- Other monthly items.
What makes up the per-minute cost?
1. Retell voice infrastructure ($0.055/min) cost - You pay this regardless of which AI model or voice you select.
2. The LLM model cost - The LLM (GPT 5.5, Claude 4.6 Sonnet, Gemini 3.8 Flash, etc) decides what the agent says. This is normally the component with the greatest pricing variation.
3. Voice provider (TTS Cost) - Voice provider (Retell Platform Voices, OpenAI voices, ElevenLabs voices, etc.) converts the AI’s response into speech.
Note: ElevenLabs is the only outlier at $0.040/min, about 2.7× the flat $0.015/min every other provider charges.
4. Telephony (Retell Twilio/Telynx or Custom Telephony) cost - Retell’s telephony line item is $0.015/min. Retell's country selector indicates that the precise rate can depend on the country and carrier.
Note: Retell does not disclose what an external carrier may charge when you use custom telephony; it only shows the cost of Retell’s telephony line item.
5. Call Agent Add-ons - Knowledge base, Advanced denoising, Safety guardrails, PII removal, AI Quality Assurance.
*AI Quality Assurance: first 100 minutes are free, then $0.10/minute.
Note: Before removing an add-on purely to reduce cost, decide your safety, privacy and call quality requirements in advanced.
