Jherik should haggle with them but a going rate would be a silver per room. The entire group can have four of the rooms but two of the rooms are being used by the carpenters right now, so that doesn't leave much left. He will give them a key ring with the keys on it. As the rooms are being prepared, the party can sit and talk and have a hot drink. At one point the tavernkeeper should comment to his wife that he had lost his spare set of keys.
Some workers are in the corner talking loud enough for the party to overhear.
They are discussing how the big treasure had to be hidden there someplace behind one of the walls. They will do more searching that night. The carpenters will stop talking if they notice the players overhearing them but it is important that the players do hear them. Their names are Boris, Grizelda, Uther and Benjamin. Benjamin is the leader.