It won't be free. Microsoft has already proven that people will pay for this service; and Sony is a business, not a charity.
Not to mention the basic fact that it costs money (in hardware and monpower) to run the centralized service, so if it were free, Sony would actually lose money. The only way to make it free is to do what they did with the PS2, which is basically to follow the PC model: Offer networking hardware and protocols, but let the game developers or other parties create the game matching services if they want. Personally I prefer this model.