There is a small cost (I think < $300) for their side of an application (i.e. 457), but nothing more than that. The major onus on an employer is proving in documents they provide sufficient training to Australians and have the size of their budgets for Australians. Also they have to be at least a minimum number of employees (I think 10) to be able to sponsor someone.
And something to do with the fact that they need to prove they've tried their best to advertise and hire residents? And I read somewhere that there's some minimum wage to hire non-residents so employers can't abuse the system.