$3 for 80 units and yet there is always an offer that's $5 for 735 units. Why would anyone buy the piggy bank
You can only buy the collectors card once a month. So while it has huge value, that value is capped. People still buy Odins, even though Odins offer much less value (31 units per $1 USD vs 147 units per $1 USD), because they want to spend more than $5 a month and get more for that money.
There's lots of reasons why someone might buy the piggy bank. They might need the potions, and decide to buy the piggy bank as a path to get them. They may be playing casually and want to spend a small amount today for stuff today, and not wait a month for the full benefits like with the Collector's Daily Card or Sigil. They may spend so much already that three bucks to crack a piggy bank is chump change so why not.
Efficiency is only one criteria people apply to their spending, and most people don't play the game remotely efficiently anyway.
Comments
There's lots of reasons why someone might buy the piggy bank. They might need the potions, and decide to buy the piggy bank as a path to get them. They may be playing casually and want to spend a small amount today for stuff today, and not wait a month for the full benefits like with the Collector's Daily Card or Sigil. They may spend so much already that three bucks to crack a piggy bank is chump change so why not.
Efficiency is only one criteria people apply to their spending, and most people don't play the game remotely efficiently anyway.