The Maria Mixteca actually has a few budget rooms that are accessible a few doors down from the actual hotel. They're cheaper and the bathrooms are smaller, but I'm told that they're otherwise not too different. I stayed in one of these for a couple of nights, and thoroughly enjoyed it. The room was clean, air conditioned, had hot water, and was conveniently located right by the plaza. I would stay there again.

The best thing - air conditioning! Quiet and effective. In the hot, humid climate of Huatulco, it's a god-send.
The worst thing - wi-fi that crawls (during high-usage times of day). Needs a major increase in capacity.