Plastic eyeglass frames and a standard lens blank, possibly integrated, should be very inexpensive to build in volume. The primary cost is likely to be in making and possibly in installing the prescription lenses. What is needed is a low cost, field customizable lens.
I've found frames on alibaba.com that only cost $0.50. Like you said, the trick is finding prescription lenses that are very cheap, and that could be made to fit a certain frame. Thanks for the comment!