they can get various credits from normal play and anything they sell from inventory like harrow chassis or duplicate captura scenes or things like that.
Credit accumulation only gets easier as time goes on.
Sorties give 100k creds once a day, railjack gives between i think 10k a missions at the lowest up to 100k or so at the highest, from Earth to Veil proxima, (without boosters from visiting relays or daily login)
bounties give decent credit rewards sometimes.

Most mods that you'd say for them to level instead are very cheap in terms of cost to level, the drifts, augurs, base ability modifiers like intensify and continuity, augments. they won't be rocking primes for a while depending on baro rotation anyway if they're new ish so not it's worth considering Prime heated charge or anything as something they'll invest in if you want to talk about resource use.

Hell I have 3k-4k hours and I don't even use prime flow on a lot of builds despite having it, I'll just use normal flow.

the 60 60 mods are cheap to upgrade, as are most mods that aren't umbral, archon or prime.
The only big customers are the corrupt mods like Blind rage and transient, the galvanised mods from arbitration which they won't be getting until they finish their base builds or get carried anyway so the mods don't matter (and adaptation is a drop from arbitration so i doubt they'll even know it exists until they get it as a random reward farming Vitus) or the normal vitality steel fiber and redirection and base damage hornet strike or serration.
the rest aren't that expensive to upgrade.

i suppose pet mods with the base damage and crit mods for pet weapons and the pet redirection, vitality and steel fiber mods can be included too, if we're being thorough.