The Macau Hoteliers and Innkeepers Association with support from the Macau Tourism Board seems to have recently launched a new hotel booking website that features many of Macau's budget (low-cost) hotel and guesthouses. See link.
The website seems to be in a beta stage at present so it might take some time before it is fully functional. However, many budget hotels are listed along with contact phone numbers.
This website is supposed to allow direct online booking for its member budget hotels. Would be good if someone tries this and reports back on the process.