So I'll help you understand this.
ADC gold efficiency is absolutely absurd. First of all, the itemization cost of zeal items is among the lowest in the game. It's similar in cost to some support items. Truly absurd. Add into that the absolutely overloaded effects of shiv and RFC, or the stupid quantity of durability PD gives. And get this, there is a stacking movement speed bonus for every zeal item. Yeah, bet you didn't know that one. The "unique passive" on zeal, stops being unique upon upgrade.
Now, the accurate and well intentioned response to this is "Well what about BT, Maw, Deaths Dance that are among the worst expensive items in the game" and they're right. For the most part, ADCs require some AD to make full use of their attack speed and crit. But, enter here a concept called "Synergistic efficiency"
So, I'm sure as you all know, the effective health of a Target is dependent upon their health and resistance stats. More health makes resistance more efficient and vice versa. It give late game tanks a scaling advantage over bruisers and assassins. But, that's two stats that work together. What if, we could imagine a set of stats that work together in a group of three. Like, idk, Crit, Attack Speed, and Attack Damage?
So, here is a fun concept, the more attack speed and crit you have, the more efficient AD is, **BUT** unlike in health and resistance, the efficiency increase is itself exponentially growing because AS and Crit synergize as well.
So, no, ADCs do not have expensive paths, because the cheapness of items that give both Attack Speed **AND** Crit makes the expensive point for AD items so irrelevant because of how overwhelmingly efficient it becomes. I mean laughably irrelevant. Like Aatrox 0/23 with at 15 minutes kind of irrelevant.
Oh also, **DING DING** bonus round. Let's talk about distributive efficiency. Notice how for tanks, an item gives primarily health or resistance. Thorn mail is armor weighted, Visage is Mr weighted, omen is armor weighted, Sunfire is health weighted, Warmogs is all health, omen is armor weighted. As a result of this, you can not rely on several types of similar items. I have to mix it up, and take trade off at times, and sometimes can't even consider certain tank build because the armor/MR distribution in relation to when I get my health makes my build very innefficient until certain power spikes. Those spikes are often based on full items.
Now consider the distribution of crit and AS in the real items. It's nearly identical in them all. There is no relative pacing efficiency for each item, because they're all the same. Which btw, is the MOST EFFICIENT BASE COMBINATION OF AS AND CRIT. Yeah, imagine that, it stacks in perfect proportion to be efficient without power spike relveance between items to maximize each other's usefullness. The order in which Zeal items are built is not important compared to the importance in order of tank and bruiser items. I keep mentioning tanks, but only because I can't stomach the sickening feeling I get looking at the disparity to bruiser build limits and efficiencies.
FIX YOUR GAME RIOT, YOU MAKE ENOUGH MONEY IN SKINS, MAYBE ASK A FEW ACCOUNTANTS TO EXPLAIN THIS TO YOUR BALANCE TEAM.
In conclusion, AP champs scale mostly linearly. Along with bruisers and assassins.
Tanks benefit from resist health synergy and scale quadratically, but they have numerous handicaps like uneven proportionality, spike dependence, and low damage.
ADCs get Cubic scaling, because they have three stats working in their favor, all multiplicative, and those stats are hand fed to braindead ADCs who never need to consider build path because of perfect proportions. That looks like this
But don't worry, despite all this disparity, absurd efficiency, uncapped stacking (unlike CDR), and perfectly spoon fed proportionality of stats, ADC items aren't 100% busted beyond belief. Trust Riot, buy your illaoi skin.