In Koh Chang, the busiest and most popular beach is White Sands Beach (Haad Sai Khao). It is still possible to sleep for cheap teher. There are a few small complex of bungalows and guesthouses with basic but charming rooms. But they are rare. Those installed on the beach are located before and after the Rock Sand Resort. It is not easy to have a room in these places beacause the number of rooms is limited and many of the customers are regulars who come back year after year. So finding a room for less than 1000 THB at White Sands Beach is difficult except in the 3-4 accommodation across the street from the beach, which are less expensive but also often full. If you are targeting to stay at White Sands Beach, in high season, it is better to book a room in advance.
Backpackers go in general to Lonely Beach. Bungalows are cheaper but the beach is less beautiful and rocky. Hat Klong Prao with only a few cheapaccommodation and Hat Kaibae which has more options, have nice beaches, especially Kaibae, and are quieter. For Kaibae, booking in advance is better in high season. We also recommend you to stay in one of the homestays in the fishing village of Bang Bao. But there you will not have a convenient access to the beach.