From my experience (dropping 1.050 keys each rebate) i found that the real diminishing return was after all my runes were lvl 8. Basically when i couldn't lvl up 1 of each rune/position each rebate.

More or less when you start getting around half of the lvl 5 runes you used to get.

Atm where i'm in (10 ->11 on my pos 1) i get around 1 lvl up every 2 rebates.