I don't think that reducing the price of crit items will solve the main problem
Crit ADCs can't work well currently because most games are over before they even get a chance to reach their powerspike.
Also, damage is so high that they can't survive early, even with the base stat buffs that Riot recently gave.
What Riot should be doing is to nerf early game and, possibly, reduce the overall damage. I know that this sounds cliché, but it would have been a better solution overall, not only for ADCs, but for all classes, peharps for the entire game.
Reducing the price of crit items while lowering their stats for late game actually makes it so that crit ADCs can snowball as hard as other classes currently (so it doesn't solve the snowball problem; instead it just worsens the problem) while actually making their late game worse, not that they are going to make it to late game most of time, but when they do, they are weaker than before.
Another thing that Riot should have been doing is to make towers more useful and Baron Nashor an actual boss that is difficult to take down. With those changes, ADCs would be even more necessary to take those objectives, and you wouldn't have to nerf mages for going bot.
I don't know why Riot insists in this early game meta, with useless towers and a laughable Baron. It is just so unhealthy for the game.