There are a couple of budget hotels in the Abu road station area. Distance from the hotels to Mount abu is around 20kms. Better stay there. Roam around the city area. Enjoy steet food. Ride through the hilly roads and you'll love the view while driving to mount Abu or Guru shikhar. Get some beautiful snaps on the way.
I have recently stayed at Shivalik hotel, Mount Abu road area.
By the way there are some costly hotels up the hill side too..