Upwork charges freelancers a sliding fee based on the freelancer's lifetime billings with a specific client. In other words, the more business a freelancer has done with a client, the more they'll earn.
Specifically, Upwork charges the freelancer a fee of:
20% for the first $500 billed with the client
10% for lifetime billings with the client between $500.01 and $10,000
5% for lifetime billings with the client that exceed $10,000