I have been doing home improvement in Ningbo for many years. Let me list several grades.
1. Ordinary/intern designers sometimes waive the design fee in order to sign the bill.
2. Experienced designers charge 30 yuan-150 yuan/flat design fees, depending on how to talk about the complexity of the design, the price is not fixed.
3. High-end designers charge design fees of 200 yuan/Ping or above. For example, the designer Jiang Zhifen I know has a design fee of 200-300 yuan/Ping.
I hope it helps you. If you have any questions, please ask.