One theory for this is:
In most of the games, Bug types are super common early on, and lots of them know Poison moves at low levels, such as Poison Sting. So early on in the game, you're a lot more like to get Poisoned than Burned or Frozen. Next most common is Paralysis.
However, Poison deals damage for overworld steps taken outside of battle, and even fainted Pokemon in the previous generations (now the HP drops till 1, then it's automatically healed).
So it's very important for trainers to be able to use more Antidotes early on in game or risk getting whiteouts. This could be why they're cheaper, because you earn very little Pokecoins battling Bug Catchers and Youngsters, who are very common early on.