While watching episode 5 of Season 2, "The Apple," Mr. Spock steps in front of Kirk and takes a hit from poisonous barbs from a dangerous flower on the planet they are investigating.
After being administered some form of antidote/adrenaline-like shot by McCoy, Mr. Spock awakens from his unconscious state and is immediately scolded by Kirk for risking his life for Kirk's sake.
The dialogue continues as follows:
Kirk: Do you know how much Starfleet has invested in you?!
Kirk (abruptly): Never mind.
...and that is the end of that discussion.
My question is, what was Mr. Spock about to reference? It is my understanding that money is not in use at that point in future-history, so what was Spock talking about?
I know that this was still the early days of Star Trek and that in the 1960s, writers didn't care as much about continuity like we do today. But, still, I'd like to know if ever an official explanation was given regarding where that discussion was going.
How much was Spock specifically worth to Starfleet?