Though I am not advocating a strict adherence to the wealth by level progression, if that progression is used, how would "extra crew members" of PCs fit on the chart? The leadership feat text does indicate that the "cohort does not count as a party member when determining the party's XP". Does this also apply to wealth? Does a higher level PC with a cohort and followers via the leadership feat factor the equipment of the cohort and all of the followers into the wealth of the PC? Or does each individual entity track wealth separately? How does a DM figure out, if the cohort / follower(s) is brought on an adventure, what amount of treasure to reward the party with? And to a lesser extent, how does this apply to familiars and animal companions who may support certain item slots?
XP: “The Cohort does not count as a party member when determining the PARTY's XP”
To determine XP: “Divide the cohort’s level by the level of the PC with whom (s)he is associated with. Multiply this result by the total XP awarded to the PC & add that number of xp to the cohort’s total.
IF a cohort gains enough XP to bring it to a level one lower than the associated PC’s Character level, the cohort does NOT gain the new level-it’s new XP total is 1 less than the amount needed to level up. (Cohorts must be at least 2 levels lower than the PC)
Wealth: (Edited with reference) “The cohort should be equipped with gear appropriate for its’ level.” This is for "starting wealth" Once they adventure with the party they are entitled to some treasure.
DMG pg 105:
"sometimes a cohort seeks no pay, only the opportunity to serve alongside the PCs. Such cohorts require only living costs. However, they are not common."
Cohort gets half a share of treasure
See @Ben-Jamin’s answer.
Personally, I feel that Leadership is already an incredibly powerful feat. Making the Leader pay for that opportunity to double their action economy seems entirely justifiable to me. Making the feat double your action economy and add extra wealth seems even worse than it is otherwise.
Familiars, Animal Companions, Psicrystals, et al. do not get wealth
These are still NPCs, but were they not attached to another character they most likely would not have treasure, and in a few of these cases they literally cannot exist as separate characters. Any gear they wind up wearing should be paid for by the master, just as a knight would pay for his horse’s barding.