Oracles in ARAM (5 minutes with true sight for 300 Gold) is nice and all, but i average using 1200 gold (on no combat stats) per game if there is a Teemo, cause when he is in the game it always takes forever and a half for the game to end.
And the worst part is that it's not just one person using all this gold, it's usually 2 or 3 cause if you only have one they just focus that guy and then the team has to deal with the shrooms anyways. topping it off they are only worth 10 gold each, so you would have to kill 30 shrooms in 5 minutes to get the gold back.
If the team is on top of the shroom game then Teemo can just make anti minion strips, basically delaying the game for so long that you suddenly have 50+ second death timers, but Teemo's Team can't lose cause they have anti pushing shrooms while the enemy has to watch as they slowly lose.
Twisted treeline is even worse, Oracles doesn't exist there, you have your small aoe trinket thing with the 90 second cooldown, nothing else, Teemo can literally place 7-8 shrooms between trinket uses, so if he spaces them out enough you will never remove them all unless you find them with your face.