As I watch old Star Trek episodes on H & I, I'm not sure what to make of Kirk's frequent references to money: is he being humorous or had the writers not yet decided the Federation doesn't use money?
Episodes like "The Apple" and a few prior to that, Kirk tells Chekhov he's earned his pay for the week. In "The Apple" in particular, Kirk asks Spock how much Starfleet has invested in him. Spock actually begins to rattle off a large number, but Kirk stops him before he can say "dollars" or "credits" or whatever. Spock's line makes it hard to think Kirk's question was meant humorously. It also occurs to me that Spock could be tallying up man-hours or some form of energy units.
I am aware of at least one non-canon source saying Starfleet can provide officers with money for dealing with non-Federation people as required for a particular mission. This seems like a reasonable extrapolation, but I doubt the writers were thinking of such details when writing the first season of Star Trek.
P.S. I'm also wondering if anyone knows all the "heroes" and "icons" clips used on the H & I promo, I'm pretty sure there's a DS9 "Second Skin" clip in there, but Odo saying "and" could come from just about any DS9 episode. I'm not sure if that's an appropriate question to ask here. If it's not, please ignore this postscript.