With solar lanterns or houses, you can just make prefab ones and sell the same one to everyone. If you can make them cheap enough, middle men will pop up to make a profit selling them to help with the distribution problem.
Eyeglass lenses on the other hand, need to be made for a person and often changed every couple years.
The problem is not making $2 glasses, but determining the prescription of 1 billion people with limited access to an optometrist.
Adaptive Eyecare[1] has done some interesting things with liquid lenses which solves both of the problems for people with spherical ametropia. Unfortunately, it won't work for people with astigmatism which is quite common (upwards of 30% in people over 30 according to some studies).
A portable device that allows someone with minimal training to determine a person's prescription, even if fairly expensive, would go a long way to solving the problem.