Roger,
Here is my take on cost-
I think they are cheap for several reasons. First, they are made in China. Second these light have a single LED and it is not a top-quality unit. They are second or third sorts from production. The battery is a NiCd not a nickel metal hydride or a lithium. The capacity is very low- it only has to supply one LED for 5-8 hours. Nobody cares if they go dead at 3am. The solar cell is also anemic. It only needs to charge the very small battery and it has typically 10-12 hours of daylight to accomplish this feat.
Bottom line, all the components are very small, low capacity, and made by the gazillions in China.
OTOH, batteries and solar panels that we want for our homes and sailboats are high-capacity, top-shelf items.