It doesn't have to be peak season to be that expensive. Regular rooms at a deluxe? Yes, probably peak season or GF, but when you have more than 4 adults (because really, who wants to sleep on a little daybed?) you're talking that much money for a suite or DVC rentals. All of the deluxe options we looked at for our value season trip were in that price range, and we looked at everything from 1 & 2 bedroom DVC villas, club level suites to 2 regular rooms. All the rack rates are in that range, with some being tons more.
Now, actually paying that much is a different issue. Obviously there's tons of discounts out there and point rentals if that's your thing, but it's not hard to find rack rates that much, even in off seasons. Fact is, when you don't fit or don't want to fit in one regular room, rates get crazy fast. You need to reaaaaaally want that onsite room to not be swayed over to house or condo rentals.
Since people have mentioned it, I could never get the DVC point rental numbers to be that much different than the discounted Disney rates. Sometimes they were worse. The numbers always came out similar for us, and going with Disney gives you the reassurance of knowing you're not relying on a 3rd party. For example, a BCV 2 bedroom is $6188 for our dates ($14 a point at a rental company), and renting it from Disney with the 35% they offered for our dates is $5,395.