Comparison basis
Use the same aggregate hour as the Otto worksheet: six ten-minute sessions, 36 minutes of visitor speech, 12 minutes of assistant speech, and 12 minutes of pauses. Otto includes six final briefs and ongoing agenda analysis. Its $0.894/hour is an assumption-based API estimate, with a workload range of $0.40–$2.85. It excludes hosting, maintenance, plan minimums, and other operating costs.
The managed-platform rows use published usage/overage rates, not minimum monthly invoices. They include different operational capabilities and do not automatically implement Otto’s discovery behavior. This is a budget comparison, not a tested quality or total-cost comparison. No promotions, free credits, enterprise discounts, or phone charges are used.
Managed platforms
| Option | Published pricing basis | Cost per aggregate hour |
|---|---|---|
| Otto, current default routing | Estimated LLM $0.657 + STT $0.120 + TTS $0.117 | $0.89 |
| ElevenLabs Agents | $0.08/minute, including speech and hosting; LLM extra | $4.80 + LLM |
| Vapi | $0.05/minute hosting; STT, LLM, TTS and transport extra | $3.00 + providers/transport |
| Retell, platform voice + GPT-4.1 mini | $0.055 infrastructure + $0.015 voice + $0.016 model per minute | $5.16 |
| Retell, platform voice + GPT-4.1 | Same infrastructure/voice + $0.045 model per minute | $6.90 |
| Speechify Agents | Starter $0.075/min; Pro $0.070; Scale $0.068; LLM/STT/TTS included | $4.50 / $4.20 / $4.08 |
Sources: ElevenLabs pricing, Vapi pricing, Retell component pricing, Speechify pricing.
For an illustrative budget using the same upstream AI spend, ElevenLabs plus Otto’s $0.657 LLM allowance would be $5.46/hour. Vapi plus Otto’s full $0.894 AI allowance would be $3.89/hour before transport and other extras. Those additions are our scenarios, not vendor quotes: model routing, integration compatibility, background work, and actual billed usage have not been tested on either platform. Bringing API keys moves provider charges to those providers; it does not eliminate them.
Retell’s listed examples cover its conversation configuration. Replicating Otto’s independent analyst, confirmation rules, and reviewed briefs would require additional implementation and potentially extra inference. Its knowledge-base add-on is $0.005/minute, or $0.30/hour. ElevenLabs speech within Retell is a different purchase from ElevenLabs Agents: replacing Retell’s $0.015 voice with its $0.040 ElevenLabs voice adds $1.50/hour. Retell pricing
Direct OpenAI Realtime
OpenAI is an API building block, so it is a closer comparison to assembling Otto than to buying a complete managed agent. Native audio replaces the conversational LLM/STT/TTS path; do not add Otto’s entire speech stack on top.
| Model | Audio input/output per million tokens | New audio only, for this hour |
|---|---|---|
| GPT-Realtime-2.1 Mini | $10 / $20 | $0.504 |
| GPT-Realtime-2.1 | $32 / $64 | $1.613 |
These are partial audio costs, not complete session estimates. Sources: Mini pricing, full model pricing.
Using the documented audio tokenization:
New user audio: 36 minutes × 600 tokens/minute = 21,600 tokens
New assistant audio: 12 minutes × 1,200 tokens/minute = 14,400 tokens
Mini: (21,600 × $10 + 14,400 × $20) / 1,000,000 = $0.504
Full: (21,600 × $32 + 14,400 × $64) / 1,000,000 = $1.6128Additional charges include text, reasoning output, conversation history, tools where applicable, and optional input transcription. Earlier turns become input again; caching reduces that charge but does not make it zero or guarantee a cache hit. Empty audio can be filtered by voice activity detection. Measure actual response usage with the intended prompts and workflow. Realtime billing mechanics
As an explicitly hypothetical adaptation, retaining Otto’s agenda, checks, and report budget adds $0.504/hour: its LLM budget minus its conversational generation budget, both including the 20% reserve. This gives $1.01 for Mini or $2.12 for the full model before Realtime text/history/reasoning, transcript provision, and hosting. This is still incomplete, not a deployed alternative or a claim about how much supervision these models require. It shows why we should evaluate direct Realtime rather than claim every alternative costs five times as much.
Monthly bills and the build decision
Included allowances and unused capacity change effective prices. ElevenLabs Starter is $6/month with 75 minutes; Speechify Starter is $10/month with 120 agent minutes. Higher Speechify plans cost $99 and $499 monthly. Allocate the actual plan fee plus overages across usage; never charge included minutes again. Otto also uses Speechify’s paid TTS allowance, so its own minimum invoice can exceed modeled usage at low volume. Sources: ElevenLabs, Speechify.
At 100 aggregate hours a month, Otto’s base API estimate is about $89. The illustrative ElevenLabs-plus-same-LLM budget is about $546 in usage equivalents. That roughly $456 gap must pay for any extra hosting, maintenance, and engineering ownership before becoming a saving. For example, $1,000/month of additional custom-system ownership would require about 219 hours/month at that gap, before other differences. This is a sensitivity calculation, not a staffing quote or a forecast.
The article should lead with Otto’s outcome: a maintained discovery agenda, corrections that invalidate stale decisions, explicit confirmation, and a reviewed brief. Lower estimated API spend supports that choice. It does not establish that building a bespoke voice stack is the economical default for a low-traffic website.