Some oracles are more expensive, especially the ones with more cards. But I think the really expensive decks are OOP (out of print) and so their value is how desperate someone is for them or would be willing to pay. The Philosophers Stone (which I think I would like in my collection one day) I have seen for very high prices and has been OOP for a long time. Also people on AT have started seeing high prices for the Tea Leaf Oracle because it was OOP and becoming hard to find. It's original price was about £40 (it has a lot of cards and is a huge box) but it went for 3 or 4 times that. They have just announced a reprint and new version so the people charging rip off prices are going to now have to reduce them
Also seen the oracle of the radiant sun and moon oracle for crazy prices but all have been OOP for a while.
If a deck you want is OOP, Amazon is a bad place to look because people deliberately inflate the prices hoping someone will pay that. Better to search through Ebay (you can set up a search that emails you for your title and sometimes you can catch a great deal!) or there is second hand book sites like Abebooks in the UK. Also if you are a subscriber here, you can post in the trading area that you are seeking a deck, often people might have a spare or one to give away or sell or trade for something you have.
But there is always going to be decks you wish you had got but are now a crazy price and there is always going to be decks you hate or you feel cold towards, that you can't believe how much they are