Upkeep is the amount of money paid by a side every Turn to retain the services of a Unit and cause its rations to pop.Erf-b1-p039 and Erf-b1-p138 Upkeep is paid every turn, but does not need to be paid in full. By consuming provisions, only a portion of a Unit's upkeep needs to be paid.
The figures above seem to be the two extremes: Stanley is disgusted that even a Chief Warlord could cost over 1000 per turn (suggesting it is one of the highest he has encountered), and Parson seems shocked that Decrypted Ansom would have no upkeep at all. It is quite possible that the upkeep for regular uncroaked and other low-level units might cost 0 or near 0 schmuckers.
It is not known if any upkeep goes to the Unit itself (as a wage), or if it all simply disappears.