"However, perusing the HD Ryobi page, they also have a string trimmer combo with included battery (same 4ah) AND charger for $150. What? That's twenty ducats cheaper than just the battery. "
Not uncommon at all, unfortunately. I've seen that with many, many products over the years, from tools to kitchen wares.
You're telling me that the slide in baking pan that came with my stove, and which I'd like to replace, is nearly one quarter of the cost of a new stove (well over $100 at that time nearly 25 years ago).
Or, how a lot of times it's cheaper to replace the whole goddamned ink jet printer than just replace the ink cartridges?
Yeah, doesn't make sense, does it.