I don't know if oracle decks are more expensive, tbh. Tarot decks seem to have similar prices. I guess the more cards in the deck, the more expensive the deck gets. But that's pure logic (I recognized as soon as I typed it, lol). It also depends on how rare a deck is, some are constantly sold out.
I ordered two decks that the supplier promised to look for through the world, so it's only a 50/50 chance of actually getting them. I'm sure it's a marketing thing, they never failed me to this day, but I expect they will at some point. Doesn't hurt to still have hope
I do believe in tarot/oracle cards but not in the way of "ooh, they are magical and foretell your future". For me it's more of a tool to speak to the divine and the higher self to aid me through life. The magic is actually within the user, not the cards. But even I am skeptical at times. It's perfectly normal. I always take my readings with a grain of salt.
You can't really believe the cards will tell your future and it's set in stone, that's not how it works. And people tend to believe exactly that.