If you are talking about Priceline's name your own price where you don't know the name of the hotel, of course it can be less money. You are comparing apples and oranges.
If Hilton was to make available to members at a lower price, nobody would pre reserve at the higher price, or they would all cancel their higher priced rooms a few days before and take the lower priced rooms if they were available.
Hilton is in the business to make money, not to give rooms away at bargain basement prices. They want members to pay as much as they can get them to pay for the room, and if they have excess inventory, they will dump it through Priceline where they don't have to offer any points or benefits for the room.