Ok, here is how to actually fix adc's

Reducing the gold cost on adc items is really not the right path. Your putting them right back to the hardest scaling (albiet nerfed version) class with the cheapest gold build.

So for example. IE can be purchased out of a BF sword and a new item which costs 1500 gold, gives 20 ad, 10% crit and increases crit damage to 150%.

Zeal upgrades all lose 10% crit so you don't pay for wasted stats. Maybe give them a tad of ad, like 10 a piece and increase the attackspeed by a bit.

The goal of adc changes since they don't scale too fast right now should be to smooth out the curve in the middle. Which means they need sort of a Midway build where they are effective in the midgame and can build that up to late game.

Think IE component double zeal and LW for 40 ad 50% crit. Crit increased to 150% damage and 10% armor pen. And as you start finishing up those items then the late game build comes together.

Just my two cents. But I don't really play adc often so I am not sure how that will affect those crit adc's.