Cost Comparison of Text-to-Speech Models for Voice Agents

Voice agent developers face significant challenges in managing the costs of text-to-speech (TTS) models, which represent the most expensive component of the typical cascaded architecture used in production: speech-to-text, large language model, and text-to-speech. A recent analysis highlights the complexities of comparing TTS costs due to varying character-to-minute conversion rates, hidden fees for concurrency, and differences in performance metrics.

Key TTS Providers and Cost Metrics

According to the analysis, only a small number of TTS providers meet the naturalness and responsiveness requirements for production-scale voice agents. These include AssemblyAI, ElevenLabs, Cartesia, Rime, and Fish. Each provider offers different pricing structures and performance characteristics, making direct comparisons challenging.

To standardize cost comparisons, the analysis uses a conversion factor of 600 characters per minute, derived from extensive production data. The normalized per-minute costs for each provider are as follows:

• AssemblyAI-TTS-1: $15 per million characters ($0.009 per minute), unlimited concurrency, 90 ms Time to First Audio (TTFA).
• ElevenLabs v4 Turbo: $40 per million characters ($0.024 per minute), ~$10 monthly spend commitment per concurrent call, 190 ms TTFA.
• Cartesia Sonic 3.6: $37.50 per million characters ($0.0225 per minute), ~$10 monthly spend commitment per concurrent call, 285 ms TTFA.
• Rime Coda: $50 per million characters ($0.03 per minute), unlimited concurrency, 395 ms TTFA.
• Fish Drama 3: $50 per million characters ($0.03 per minute), ~$10 monthly spend commitment per concurrent call, 370 ms TTFA.

Concurrency and Hidden Costs

Concurrency limitations and hidden fees are a significant pain point for developers. Most TTS providers require annual contracts to unlock production-scale concurrency, with an average spend commitment of $10 per month for each concurrent voice agent call. This can result in substantial upfront costs for scaling new applications, as unused commitments are non-refundable.

In contrast, AssemblyAI offers unlimited concurrency without requiring spend commitments, allowing developers to scale their applications without hitting concurrency ceilings or negotiating additional contracts.

Performance and Developer Experience

AssemblyAI-TTS-1 stands out for its combination of low latency (~90 ms TTFA), competitive pricing ($0.009 per minute), and unlimited concurrency. These features make it a compelling choice for developers seeking to optimize both cost and performance in production environments.

By addressing the complexities of cost normalization and eliminating concurrency-related barriers, AssemblyAI aims to simplify the deployment of voice agents at scale, enabling developers to focus on building their products without operational bottlenecks.