The building will likely cost more than $300, but when you divide the cost by the number of people living under the roof you could reach the $300 mark, especially using a bare-bones model.
This model is designed for expansion. The bullseye in the middle of the building can collect and distribute water through the house via tubes laid in the walls. Adding more rings to the building allows you scale the building up in size incrementally as need to provide the people working there work space, schools, day care, gardens, kitchens, more water tanks, more storage space, etc.
A foreign company could build one of these units big enough to house any number of people. Then the company could give all the employees homes and jobs at the same time. Plus, the company would have an instant work force with a vested interest in the success of the business, especially if given a profit sharing incentive to upgrade the building. When the workers retired they'd already have a retirement home and the company could build another office for pennies.
If the walls are reinforced with wire mesh and concrete the structures could last for generations and allow its occupants access to a school, a job and a retirement home at birth. Workers from different buildings could swap houses and jobs with people in other buildings built using the same model.
You could even take the idea a step further and instead of filling the concrete walls with sand bags and dirt, leave them hollow. Then put a deep, hollow base at the bottom and you could make a stationary floating live-in eco-island office. Wealthy companies in cities like Singapore could build these for upper class workers moving them out of the city thus lowering the demand for real estate and making property prices more affordable for those onland.