Is it necessary to apply for a patent for a utility model that is easy to be imitated by small factories? Does the country have any good policies now?

It is still necessary. After you have a patent, you can sue others for infringement by imitating your product to the local intellectual property office or the court. The incentive policies for patents vary from place to place, so you have to notify the local intellectual property office.

As for the agency fee, if practical, it is roughly between 1000-3000.