I haven't fine-tuned GPT-3.5-turbo yet, but overall llama2 is probably going to be cheaper. Especially if 13B is good enough for you.
You can get roughly $0.003/1K tokens using modal.com for inference on a fine-tuned llama2 13B model, and there's definitely cheaper options available.
Unless you have a ton of data, the cost for training llama2 is usually a few dollars.