I would gladly trade the +%AP buff (PBE) on Rabadon's for a lower cost.
As it stands, Rabadon's is the most expensive item in the game. More than IE as well as Triforce. It doesn't feel good to use/buy and I often find myself purchasing anything besides Rabadon's because of how poor the item feels.
It's okay if the item becomes less cost efficient as long as I can buy it sooner in the game. A reduction of 200-300 gold feels warranted considering the prices of other items.
EDIT: I should clarify they're raising the amount of AP it gives you by 5% on the PBE. I'm not referring to the item's current AP buff.