posts/ideas/cost-of-switching-proprietary-to-open-weight-models.md
Table of Contents
Post Idea: The Real Cost of Switching from Claude/OpenAI to Open-Weight Models
Status: Idea Date: 2026-05-22 Source: AI-Powered Development slide series (Parts 1-3), Engineering Excellence talks, Dom/David & Dom/Vela 1:1s
Hook
I switched from Cursor (Composer 2 + Claude models) to DeepSeek Pro/Flash via ACP adapters for my personal development workflow. The result: my 4-week DeepSeek spend came to just $26.25 while processing over 1.9 billion tokens across 20,000+ requests. Here's what I learned — including why some engineers find open-weight models feel "dumber."
The Numbers (Real, Not Hypothetical)
Before (Cursor + Claude)
Team of 4 engineers.
| Item | Cost |
|---|---|
| Cursor base plan | $40/mo/person → $160/mo team |
| Cursor on-demand usage | ~$94/mo team (low because some switched mid-month) |
| Total team | ~$254/mo |
Worth noting: the $94 on-demand figure was artificially low because partway through the month, some team members had already switched away from using Cursor heavy. A full month of Cursor with Claude models would've been significantly higher.
After (DeepSeek via ACP — Personal Usage)
| Item | Cost |
|---|---|
| DeepSeek V4 Flash | $0.14/M input, $0.28/M output |
| DeepSeek V4 Pro | $0.435/M input, $0.87/M output |
| 4-week personal usage (single person) | $26.25 (with 75% promo — now permanent) |
| Pro tokens consumed | 992,532,966 in 8,679 requests |
| Flash tokens consumed | 963,728,972 in 12,043 requests |
| Reasonix session with 94% cache hit | ~$0.01-0.04 per task |
Note: DeepSeek has since made the 75% introductory discount permanent, so the price above is the ongoing rate — roughly ~$105/mo at full price is now a moot point.
Comparison with Major Models
| Model | Input cost | Output cost | vs DeepSeek Pro |
|---|---|---|---|
| Claude Opus 4.7 | $5.00/M | $25.00/M | 11x / 29x more |
| GPT-5.5 | $5.00/M | $30.00/M | 11x / 34x more |
| Claude Sonnet 4.6 | $3.00/M | $15.00/M | 7x / 17x more |
| DeepSeek V4 Pro | $0.435/M | $0.87/M | baseline |
| DeepSeek V4 Flash | $0.14/M | $0.28/M | 3x cheaper than Pro |
The Prefix Cache Multiplier
DeepSeek's byte-stable prefix caching is the hidden superpower. Reasonix achieves 94% cache hit rate by using an append-only loop pattern. Cached input drops to ~$0.014/M — effectively $0.01-0.04 per coding task.
But Here's the Catch
Not everyone on the team felt the same way about the switch.
Vela (heaviest Claude Sonnet user): Thought DeepSeek was "dumber." We dug into it and found 3 distinct causes:
- Actual capability gap — DeepSeek is genuinely less capable for open-ended, poorly-scoped problems. Pro is ~77/100 vs Opus ~91/100 on one benchmark.
- Visible reasoning — DeepSeek exposes its thinking. Watching it second-guess makes it feel dumber, even when the final output is fine.
- Tone/personality — She literally said "it feels like I lost a friend." Sonnet's voice mattered to her workflow.
David (lighter Claude user): Found DeepSeek comparable. "It's good. I don't think it's dumber."
Dom (heavy planner): Prefers DeepSeek. Uses the visible reasoning to course-correct mid-thought. Built the entire Qwestly agent stack with DeepSeek Pro autonomously.
Key Insight
The cost savings are enormous (even at personal scale — compare $26.25/month vs. the $40 + usage you'd pay solo on Cursor with Claude), but the switching cost isn't just technical — it's psychological. The model's "personality," its reasoning style, and how its output feels to read all affect productivity. Some engineers can absorb that. Others can't. The math changes depending on the user.
Tags
engineering ai cost-optimization deepseek claude openai open-weight economics productivity