Why is an obscene amount of Mushroom Kingdom coins worth so little Beanbean coins? In the usual Mario games that take place in the Mushroom Kingdom, coins are very abundant and it would be very easy to rack up several thousands. Not so much in the Beanbean Kingdom.
Maybe it's not just that the Mushroom Coins lose their value over the course of the game, but the Beanbean Coins increased in value at the same time. When Tolstar first checks the exchange rate, the prince is missing and Beanbean's capital city is under attack or in ruins already. When Peasly's bet is concluded, it's long since been rebuilt, and the kingdom is in good shape.
Shortly after you enter Woohoo Hooniversity to face off against Cackletta and obtain the Beanstar, a bunch of professors run by... except for one, who falls down, tells you that the professors from the Hooniversity are being transformed into monsters, and transforms into a Laser Snifit. After Mario and Luigi defeat him, he doesn't change back. Neither do any of the other monsters in the Hooniversity. They all simply disappear, implying that they die like the normal enemies. Now think about it. If all of the monsters in the Hooniversity were once professors, then that means you were basically forced tokill all of themso they wouldn't suffera fate worse than death.
When Mario gets sick via eating an Invincishroom in Little Fungitown, the Toad doctor points out the cause as Bean Fever (It's a disease that affects people not accustomed to the food in Beanbean Kingdom) and that if Luigi doesn't get the cure, Mario will turn into a bean. Said cure is Crabbie Grass that seems to be hard to come by. Then you realize that there are a near unlimited amount of beans in the kingdom.
The aformentioned Fridge Brilliance regarding the Mushroom Kingdom - Beanbean Kingdom exchange rate becomes this when you realize that actually happened in real life to Zimbabwe. The bank actually printed out hundred trillion dollar bills, and you still could barely buy anything with them.