Marubu resort has mud baths. I think it is around $40.00. They have a website. Type Maruba into yahoo and it will come up. I am planning on trying it when I get to Belize. I think Maruba is 30 miles away from the airport in Belize City.
Maruba Spa, off the Old Northern Highway in Belize District, is the place that promotes mud baths. Chaa Creek just west of San Ignacio has a new spa facility and is probably the most complete and professional spa in Belize. Royal Mayan in Benque is another spa. Rosita Arvigo's little place in Cayo also offers some spa-like treatments.
I stayed at Maruba for a couple days in May. My friend and I had the mud massage. It was a typical full body massage, but rather than lotion or oil they used mud. The ambiance there is incredible and made it a magical experience. After the massage we were led to the bathhouse where a woman poured buckets of water over us to rinse the mud away. The first splashes startled the frogs that were resting on the wall and they scampered away. It was intoxicating.