Pantheon is only balanced in top lane by his ability to eat his entire mana bar in a full rotation. If Riot wanted to make it easier for champs like Panth and Jayce to be good, AD mana itemization wouldn't be so awful.
Amumu's Curse of the Sad Mummy cost is so high because Riot wants him to use it at the beginning of the fight rather than as a clean up. This way, if you throw your Q (100 mana) plus spam your E and keep your W up, your ult becomes unavailable even in the late game.
Karthus's ultimate costs so much to emphasize its power during his passive, when it costs 0 mana and when you should be using it anyway.
Singed's ultimate grants him mana regeneration. If you made the cost 100 it would literally be the same as making it cost nothing.
Teemo's Noxious Trap is pretty much the only consistent source of mana use in his kit. His Blind and his Ult are both very overcosted because well basically Riot wants him to be manaless with his E and his W so they have to make his mana bar drop somehow.
If you need an explanation for why Twisted Fate's Destiny costs 150 mana in the early game then you clearly haven't been here for very long. Before Twisted Fate's Loaded Dice passive was reworked, his global passive and global map pressure made him pretty much 100% pick/ban in Korea.
Vi's ultimate is supposed to be used as an engage, so it follows the same logic as Amumu's. These abilities are far too powerful if used after you've already come ahead in the fight, they prevent narrow escapes and punish immobile champions.
Warwick - Again, Warwick is an old champion like Teemo who has very little mana usage in his kit. His Q and his ultimate are overcosted because one of his abilities is a passive and one of them is a buff that costs almost no mana and has a very long CD.
Zilean's ultimate is overcosted from a relic of a bygone era where he could bring it back up every 30 seconds.
Zyra's Stranglethorns has the largest AOE knock up in the entire game (and knock ups can't be reduced by Tenacity). I think 140 mana is too low, to be honest.