I am working under the assumption that a GIANT corporation like Amazon could get it built for much cheaper then a guy in his garage.
There are limits to economies of scale, no matter how big your company is. The underlying materials and manufacturing costs have lower boundaries, and that floor for battery technology costs is likely higher than you imagine. But, OK...suppose Amazon could add that functionality for *half* of what the Battery Boots cost to make, and still get the same ROI as the current Echo. If we estimate that based on the anticipated retail prices of the Battery Boots you're still adding $35-$50 (or ~20%-28%) to the price of the Echo.
I would rather have had it as part of the echo for $200-$220 and be a warranteed by Amazon piece of equipment than a thrd party add-on.
What about the rest of us who don't want/need that portability? Adding at least $35-$50 to the cost for a feature that most potential customers aren't going to use would be dumb. Now, Amazon offering the same type of accessory as an optional add-on would make sense...but not building it into every unit.
It is a speaker so it would have made sense for it to dock.
Characterizing the Echo as "a speaker" is a bit like characterizing your PC as "a monitor".