The Secret Service pays rent to the Clintons and both Bush's. They paid rent to Ronald Reagan and will pay rent to the Obamas. It's the policy of the agency to pay for space on the property used by the Secret Service. They determine how much they will pay, not the former presidents. It's just the way things are.
I wonder if this is how the Obamas are affording all that real estate. With rent payments from the USSS.