One of the first items that might seem egregiously expensive in Hollow Knight: Silksong is the Simple Key. You’ll see this upon opening the merchant's shop in Bone Bottom, and think to yourself, “500 ...
Simple keys in Hollow Knight: Silksong are used to open doors scattered around Pharloom, but they're few and far between. As you're exploring Silksong, you've probably come across a few locked doors.