tl;dr: +1 Rob Gropper's comment regarding product market fit, but let's try and answer Brad's question.
For a moment though let's assume Brad's tested enough prototypes to establish product market fit, and now he's trying to figure out how to take his prototype to 100 million plastic units.
Materials (plastic, cheap):
- HDPE - High Density Polyethylene
- PP - Polypropylene
- PS - Polystyrene
- ABS - Acrylonitrile butadiene styrene
Manufacturing Processes (plastic, high volume):
One of the materials and manufacturing processes I've listed will likely work best for your project when you're ready to produce high volumes. However, like many people have mentioned in this thread, one of the most important requirements for high volume plastic manufacturing is the risk associated with the high cost of tooling (actually making the molds that will be used in producing your parts).
Doing everything you can to validate your design and establish product market fit will help bring down the risk of tooling. This could involve aggressively iterating on the design to make it as cheap to manufacture as possible and/or mean finding a useful sales channel for your product.
Hope this has been helpful!