It depends on how far behind you believe the model-available LLMs are. If I can buy, say, $10k worth of hardware and run a sufficiently equivalent LLM at home for the cost of that plus electricity, and amortize that over say 5 years to get $2k/yr plus electricity, and say you use it 40 hours a week for 50 weeks, for 2000 hours, gets you $1/hr plus electricity. That electrical cost will vary depending on location, but let's just handwave $1/hr (which should be high). So $2/hr vs ChatGPT's $0.11/hr if you pay $20/month and use it 174 hours per month.
Feel free to challenge these numbers, but it's a starting place. What's not accounted for is the cost of training (compute time, but also employee and everything else), which needs to be amortized over the length of time a model is used, so ChatGPT's costs rise significantly, but they do have the advantage that hardware is shared across multiple users.