IIRC the gearbox is about 1/2 the total weight of the assembly, roughly 8oz motor, 8oz gearbox. However, I consider this a module, not a component. That is to say, use it as it is, don't bother modifying it, or just pay more up front and run the OMFGBaneBots. These drills are all made in china for the lowest cost possible, and no matter how much work you do to them to make them "better" you are still polishing a turd.
That being said, rebuilding the gearbox with good internals is a great project for learning how to use a machine shop, but if the end goal is to make robots and make robots that don't fail... these may not be a good option.
THAT being said, i still think these are about the best value for money. They work, work well, are light and powerful, can be mounted with simple hand tools or off the shelf stuff from the hardware store. Stick a small wheel right on the shaft, run as close to nominal voltage as you can, break in the brushes, and have spares, they will serve you well.