There are 27 possibilities listed on http://www.hostelworld.com
(Don't be put off by the word 'hostel' within that URL. Simply use the 'Additional search options' to select both 'Guesthouses' and 'Budget Hotels).
The My Thai Guesthouse gets good reviews, with prices from �3.51 per night. There are plenty of other places, with good reviews, well under a tenner per night.