ElevenLabs vs. Cartesia vs. StackVoice: The Developer's Cost Guide
Building human-like conversational voice agents (using platforms like Vapi or Retell AI) has become one of the fastest-growing trends in AI software engineering. However, once you deploy a voice agent to production, you quickly run into a major hurdle: Text-to-Speech (TTS) API bills are extremely high.
For any conversational bot, TTS is typically the most expensive part of the stack, often costing more than the LLM (Large Language Model) token costs.
In this guide, we will compare the cost, latency, and performance of the top three options: ElevenLabs, Cartesia, and the upcoming StackVoice API.
1. ElevenLabs: The Quality Standard (But at a Premium)
ElevenLabs is widely considered the gold standard for emotional range and voice realism. Their clone engines are state-of-the-art.
- Pricing: $0.15 to $0.24 per 1,000 characters (on standard tiers, which scales to $150 to $240 per million characters).
- Latency: 250ms - 450ms.
- Best for: Audiobook generation, video voicing, and high-end narration where emotional inflection matters more than real-time latency.
- The Problem: For high-volume call centers or interactive avatars, ElevenLabs is financially unsustainable. A single 10-minute phone call can consume 10,000 characters, costing up to $2.00 in audio synthesis alone.
2. Cartesia: Optimized for Real-Time Conversational Speed
Cartesia (Sonic model) is built specifically for real-time applications. It focuses heavily on minimizing Time-to-First-Byte (TTFB).
- Pricing: $0.015 per 1,000 characters ($15 per million characters).
- Latency: 90ms - 150ms.
- Best for: Live phone agents and real-time multiplayer gaming interactions where speech needs to start instantly.
- The Problem: While 10x cheaper than ElevenLabs, $15 per million characters still aggregates to thousands of dollars per month for high-throughput automated support bots processing millions of lines of conversation daily.
3. StackVoice: The Open-Source Alternative (90% Cost Reduction)
For developers looking to eliminate vendor lock-in and cut costs to the absolute minimum, hosting open-source models is the answer. StackVoice API is our hosted wrapper running the lightweight, state-of-the-art Kokoro-82M engine on high-frequency CPU instances.
- Pricing: $4.00 per million characters ($0.004 per 1,000 characters).
- Latency: 120ms - 180ms.
- Best for: High-volume voice agents, automated customer outreach channels, and cost-sensitive applications.
- How it works: By hosting the highly optimized 82-million parameter Kokoro model on low-overhead CPU thread execution (instead of expensive GPU clusters), StackVoice achieves comparable quality to Cartesia at a fraction of the operating cost.
📊 Summary Comparison Grid
| Feature | ElevenLabs | Cartesia | StackVoice (Kokoro-82M) |
|---|---|---|---|
| Price per 1M characters | $150.00 - $240.00 | $15.00 | $4.00 |
| Average Latency (TTFB) | ~350ms | ~110ms | ~140ms |
| Recommended Use | Videos & Audiobooks | High-speed Phone Bots | High-Volume AI Agents & Startups |
⚙️ How to Transition and Save
If you are running voice agents via Vapi, Retell, or custom WebSockets, switching is simple. You can configure your prompt pipelines to route to different API endpoints based on the task:
- Use ElevenLabs for high-end outbound marketing calls.
- Use Cartesia or StackVoice for quick, conversational support chats where latency and cost are key.
We are currently running a private beta for StackVoice API. If you want to benchmark your own call volumes and get free API credits, register your email in our pricing simulator page!