If an agency is entrusted to handle it, a service fee will be charged. There is no standard for the service fee, which varies from region to region, about 2000-3500 yuan. The specific payment method can be negotiated with the agency.
I don't quite understand what you mean by "applying for a patent that nobody uses".
Patent application needs novelty and practicality. If your patented technology has been made public in the market, it will lose its novelty, and even obtaining a patent certificate has no substantive significance.