What is the going rate for brokerage commissions for a commercial sub-lease in san francisco?

It depends on a few factors. Lease commissions are sometimes based on percentage of lease payments and sometimes it's a dollar amount per square foot. The commission offered depends greatly on the strength of the market. For example, in a weaker market a larger commission is offered to the procuring broker.
If you can give me the following information then I can give you a specific answer:
-Where is the building located. If you don't want to reveal the address then just the area (e.g. Soma, China Basin, etc.)
-Type of space (e.g. traditional office, edgy tech office space, etc.)
-Type of building (High-rise office, mid-rise, converted warehouse, etc.)
-Quality of building (Class A, B, C)