We have just returned from a week at Mafia Island Lodge and had an
excellent time diving with Mario at "Big Blu", the resident dive school.
It is the cheapest of the three lodges on the island, genuinely friendly
and helpful staff.
You may be able to save a little money by staying with one of the local
villagers (I can give you a contact of someone at the Marine Park
headquarters who would welcome lodgers) but the accommodation costs at
the Lodge are extremely reasonable.  When you add the cost of flights,
the Marine Park levy and the diving, I do not think you would save too
much.  I think all the dive operators are attached to the lodges.
If you contact me privately I can give you more details.