The fee is computed based on the market conditions, the job compensation, and the job's geographic area – it may vary based on these categories. Once you have posted a job and selected a hiring method, it will show you the calculated amount to pay.
A temp agency uses the social model of employer/employee. In legal terms, the temp agency provides the employer with an employee for a specified length of time under a set of conditions as predetermined by law (replacement of someone) responsibility for whom lies with them and they agree to work for the employer.
Freelancing, however, is a contract specifying services between two totally independent professionals (economic entities). The legal framework is therefore subcontracting.