Sonnet vs GPT-6.1 Sol: Read the Chatbot Receipt

01 / Choose Claude Sonnet 5.5 or GPT-6.1 Sol by the bill

A chatbot model does not become cheap because its input price looks small. Your bill also depends on response length, conversation history, cache activity, and repeated requests. Choosing between Claude Sonnet 5.5 and GPT-6.1 Sol means comparing the cost of completing the same customer task, not merely comparing two headline rates.

Start with a receipt you can audit. The prices below are illustrative arithmetic inputs, not verified CheapRelay prices, provider quotes, or claims that either model costs less. Apply each model’s actual rates before purchasing. The workload is also a planning assumption, not a benchmark.

02 / The first receipt: 10,000 chats for $81

Assume 10,000 completed chats per month, each using one API request with 1,200 billed input tokens and 300 billed output tokens. Input includes the system instructions, supplied history, and current message. That produces 12 million input tokens and 3 million output tokens.

For this example only, use $3 per million ordinary input tokens and $15 per million output tokens. Apply the same illustrative tariff to both candidates so the starting comparison does not manufacture a price advantage.

• ■ Ordinary input: 12 million × $3 = $36.
• ■ Output: 3 million × $15 = $45.
• ■ Total monthly token charge: $81.
• ■ Cost per completed chat: $0.0081, or 0.81 cents.

These are model-token charges only. Hosting, storage, search services, payment fees, and human escalation are outside this receipt.

03 / The cache receipt: $60.90 with separate line items

Now keep the same 12 million input tokens, but change their billing categories. Assume 2 million ordinary input tokens, 2 million cache-write tokens, and 8 million cache-read tokens. Output remains unchanged.

Use illustrative rates of $3 per million ordinary input tokens, $3.75 per million cache-write tokens, and $0.30 per million cache-read tokens. Here, the write rate is assumed to be the complete charge for those tokens, not an additional surcharge. Check the actual billing convention before copying this calculation.

• ■ Ordinary input: 2 million × $3 = $6.
• ■ Cache writes: 2 million × $3.75 = $7.50.
• ■ Cache reads: 8 million × $0.30 = $2.40.
• ■ Output: 3 million × $15 = $45.
• ■ Total: $60.90, or 0.609 cents per completed chat.

The difference from the uncached example is $20.10. That is a calculated scenario difference, not an advertised discount or a promised cache hit rate. Do not assume both candidates support identical caching behavior or pricing.

04 / An extra 150 output tokens adds $22.50

Suppose either candidate averages 450 output tokens instead of 300 while everything else stays unchanged. That adds 1.5 million monthly output tokens. At the illustrative $15 rate, the additional charge is $22.50, taking the cached receipt from $60.90 to $83.40.

This is not a claim that one model is more verbose. It is the sensitivity worth measuring. At this volume and tariff, every additional 100 output tokens per chat adds $15 monthly. Ask both candidates for the same response format and measure actual billed usage.

05 / Run a matched test before routing every chat

Use the same representative conversations, instructions, tools, and acceptance criteria for both models. Record ordinary input, cache writes, cache reads, output, and repeated requests separately. Count a chat as completed only when it meets your predefined standard.

For scale, an additional 5% of requests would add about $3.05 to the $60.90 receipt if those requests had the same average token mix. Real retries may differ. Divide total spend by accepted completions, rather than rewarding a model merely for answering cheaply.

06 / Use CheapRelay to compare without a subscription

CheapRelay offers pay-as-you-go access through one key, with billing for real tokens, including cache reads and writes, and no monthly fee or subscription. Its OpenAI-compatible routes let you keep the surrounding chatbot integration consistent while testing the candidates.

Sign up for CheapRelay, check the current rates for both models, and replace the illustrative prices in these receipts. Current public rates could not be verified for this article, so there is no defensible price winner here: choose the candidate with the lower measured cost per accepted chat at your required quality.