Re: Were they paid?
Some sort of monetary system is definitely in place.
From TOS: The Apple--
KIRK: Trying to get yourself killed. Do you know how much Starfleet has invested in you?
SPOCK: One hundred twenty two thousand, two hundred--
KIRK: Never mind.
Dr. Howard, Dr. Fine, Dr. Howard: For Duty and Humanity!
--Men in Black, 1934.