What's crazy is that only a few years before you could buy the fully assembled TS-1000 for $99, I had built my first computer, which cost $106 plus extra for the address displays, and it didn't have a full keyboard, only a hex keypad, and LED displays. It had video output, but with the mere 256 BYTES of memory, the best you could do was 64x32 resolution, and part of what showed on the screen included a graphical representation of the code generating the picture. For ANOTHER $100 the following year I added the expansion board which gave it a whopping 4K of memory (still no BASIC or anything more than a monitor routine). Now you could do 64x128 graphics and not have to display the program. Approximately 2 years later, the TS-1000 appeared at the $99 price point. Leading up to that, I seem to recall the ZX-81 being offered at around $130, or $99 for the kit.
Some of the other accessories I remember are both external 'proper' keyboards and one product that clipped over the case and gave moving keys that simply pressed down the existing membrane switches when struck. There may have been an intermediate product, or maybe it was just one of the full keyboards, that required you to open up the computer and pull off the flex cable from the stock keyboard and plug in its cable in place.