It's been discontinued off and on, it seems, but finally permanently now. I'd like to personally find some of the corporat elogo OEM ones.
In general terms, the supply of old style (orange and blue shifts, brown case) ones is still okay. I've gotten a few here and there. there are a lot of warranty replacement (no manuals or slip covers) for ~$20 available right now.
The red box ones were available from some overstockers for ~$20 US in the past, but that seems to be fading. I doubt these will be costing $100 anytime soon, but $50 could happen.
I've written a few things (including a "boy scout" encryption system) for it and it's quite programmable within its limits. I'd prefer 4 real stack registers and direct access to them instead of the input/swap thing, but even as it is, it's an amazing amount of power for $20. The really early HP programmables didn't do much, if anything, more than this can.
Another one of those areas where just a little bit more would have made all the difference (16 or 20 registers, 150 or 200 steps, a better implementation of the input/swap thing.) Why do we keep getting LESS instead of more?