i wouldn't exactly call $100 inexpensive, but you're right, bluetooth speakers generally seem to be quite expensive/overpriced.
i recently got the edifier bric connect, which is about the same price but at least lets me change the 6 AA batteries when they run dry after a couple hours of playing. it's not quite as compact, but it sounds great and can get pretty loud too. it's not perfect, but it fits my need better than anything else i've seen yet.
With bluetooth speakers, these companies are essentially building the cheapest amplified speaker possible, and then add one one of those $1-2 bluetooth modules to the device. Those self contained modules can be used to pretty much mod any amplified speaker system into a bluetooth enabled one. (just feed it into the audio input).