It costs, like, $50-$60 online. Most other games only cost like $25-$45.
It probably has to do with supply and demand. As of now, Pokemon Platinum is almost 10 years old, so there aren’t as many copies being sold as newer games such as Sun and Moon. Therefore, the price of these “old” Pokémon games are generally higher than that of newer ones.
You might be looking in the wrong place. I got my copy of platinum for $15. My advice is to buy a used copy. They are much cheaper.
I even just searched "Pokemon Platinum" on Google and the first 5 listings are under 20 dollars.