Cascade — three metered legs
you pick each part; each leg has its own small bill
mic
VAD
end of speech?
STT · listen
$0.002 – $0.006 / audio-min
billed on user talk time
LLM · brain
$0.0002 – $0.0027 / turn
upper end = frontier or reasoning models
text history re-sent each turn (cheap tokens)
TTS · speak
$0.004 – $0.034 / audio-min
the biggest leg of the bill
speaker
Per conversation-minute, measured builds: $0.0037 (budget) → $0.0097 (mainstream) → $0.0252 (premium)
no-streaming latency floor 4.1 s per turn measured (LLM is the bottleneck); interruption handling is yours to build
Speech-to-speech — one model, one meter
audio never becomes text on the main path; history lives server-side as audio tokens
mic
audio in · $0.0192/min
speech-to-speech model (gpt-realtime-2.1)
listen $0.0192/min · speak $0.0768/min · WebSocket session
voice-to-voice latency ≈ 2 s, barge-in native (measured)
audio out · $0.0768/min
speaker
audio history, re-billed every response
76% of it cached at $0.40/M in our session
Per conversation-minute,
measured: $0.0573 · mini $0.0159
Measured 2026-08-17 through the Synthorai gateway · list rates at measurement time · methodology and per-leg tables in the post