My first one? I don't know. It was some sort Model M AT model that came with my 286 in 1991. You know with the large round DIN connector.
I was 11, it was my first PC of my own, and click Model M's were practically given away at the time, as the new trend where the new "QuietKey" rubber domes. Everyone wanted those because they were quiet! My computer was practically obsolete even when I first got it, and someone threw in a free keyboard.
Fast forward roughly 15 years to 2005. I had recently graduated from college and had long stopped using old clicky keyboards and switched to the modern modern rubber domes. One day I remembered how much I missed typing on a solid tactile keyboard, checked out eBay, and won a Model M in pristine condition for $8.95
This was a time before people rediscovered mechanical keyboards, so these guys were going for next to nothing on eBay.
I recently looked into picking up a similar replacement (The Model M above became a casualty of a ceiling leak at work in 2011, where I had moved it after I replaced it with a Black M13 Trackpoint II model) and was shocked how expensive they had become on eBay.
Demand must have gone up