Retail for those boxes usually range from $40-60, depending on where you live. Paying $90 for a booster box is already quite the increase.
The reason why those boxes are so high is because of supply and demand. there’s a lot of demand for booster boxes, and not enough are made and sold. Not only that, but XY series originally released years ago, so this is almost considered an antique.
Basically, it’s selling for so much because there’s not any new ones anymore. It’s kind of a collectors item.
I can’t say for sure why yours in particular cost so little. It’s possible it was already opened and resealed (unfortunately very common in the community), or someone just didn’t know the worth. Or they planned to hold onto it, but needed money ASAP so they sold it for cheaper. Maybe you got it before the price increase. I hope it makes sense as to why it’s expensive now, anyways. My advice for you would be to recheck the listing and see if there’s any notable red flags. Take a look at seller reviews, for example
I hope this helps!
Source: I’m addicted to TCG