If there is an Australian employer who will sponsor me, what fees must they pay?

The nominating employer is not obliged to pay the applicant’s travel cost or assist them with accommodation on arrival or in other ways. Some employers may offer to pay relocation costs or assist in other ways, but this is a private arrangement between them and their employee.
The employer must employ the applicant according to Australian standards and conditions.
If a change in the employer’s requirement occurs after the worker arrives, they are not obligated to employ or continue employing the worker – but the employer may have difficulty with future sponsorships. In addition, if the worker departs Australia the employer is responsible for repatriation costs.

Equally the employee is not obliged by the migration law to work for the sponsor for any particular period – but in some cases it can be part of the contract between employee and employer.