The arithmetic on a realistic month
A developer sends 5M input tokens and 500K output tokens per month. Assume 70% of the input is stable repository context and can be cached.
| Route | Input cost | Output cost | Monthly total |
|---|---|---|---|
| Claude Sonnet 5.5 direct $2 / $10 per M, 70% cached at $0.10 |
$3.35 | $5.00 | $8.35 |
| Claude Opus 5.5 direct $4 / $20 per M, 70% cached at $0.20 |
$6.70 | $10.00 | $16.70 |
| DeepSeek V4.1 Flash off-peak direct $0.15 / $0.60 per M, 70% cached at $0.003 |
$0.24 | $0.30 | $0.54 |
The pattern is blunt: if your monthly API spend on a frontier model stays under the price of a seat that includes that model, buy the seat. If you routinely exceed it, switch to BYOK and route the volume to a cheaper model. A $30 Cursor Premium seat also bundles agents, cloud tasks and code review that pure API access does not.