The reason they are so high is it has been a VERY LONG time since they were made and no longer available,
No doubt they are old and rare.
Since this thread came about, I checked an old Kenwood TR7730 2 meter mobile I have and sure enough, it used the same connector for an external memory backup power supply so it could retain its whopping 5 memory channels!
I guess the option was for those running the mobile in a shack with AC power so they could kill their 12 VDC supplies.
Knowing Toshiba used them on laptops back in the day, one would think the plug could still be found somewhere.
Possibly surplus electronics dealers. I have such a place here in town but digging through their many bins of miscellaneous plugs and connectors could take hours. I can't say I ever saw another laptop that used the same 3 pin connector. In fact, I can't think of anything else that used them other than the few devices you mentioned earlier.