When you file the application for PR in Singapore, you are required to pay the prescribed fee of 100 SGD per person in your application. So if you are filing for 3 persons, your total cost of PR application will be 300 SGD.
This cost is mandatory as the application can not be filed without this cost.
You may or may not decide to engage a consultant to assist with your application. A consultant will help you with checking all the documents, application form checks, writing a cover letter and tips for your application. Engaging a consultant is not mandatory. However, if you decide to engage, the cost for this will come to be somewhere between 1300 to 3000 SGD.