Why malzahar's W costs 90/95/100/105/110 mana?
last time i checked, he can't spam it to push you under your tower nor poking with his W and oppressing the enemy laner.
It probably cost that much because late game it easily deals 75% max health damage over the duration, which is more damage than many ultimates.
He also has free mana for days from his E, and enough burst and lockdown from the rest of his kit to guarantee someone stays in his W the full duration.
he probably doesn't have this damage at lvl 1, try again
free mana? his E gives him 0 mana at lvl 1, and 36 mana at lvl 5 against a wave, assuming he only used his E to farm try again
gains more mana, and his W/E still costs less mana while the mana cost of his Q is here to not let him oppressing you