the cost would be depending on GPU type/serving system/traffic pattern. check out some throughput comparison from vllm's blog post https://vllm.ai/
if you serve 7B on cost-optimized GPUs (A10G/L4) and keep it busy, it can be a lot cheaper than gpt3.5 turbo. tho it's not a fair comparison as 3.5's quality is still far better.