Bike magazines always seemed to do it on price?
Maybe so but how on earth would we find the price of any bike any user might enter??
Well most people, here, seem to have a handle on most of the bikes that come-up - either in (rough) terms of price, or where they were in the range.
Mid-range bikes are pretty much going to be production fodder - I'm fairly sure we all know mid-range when we see it.
Groupset it is. Easy to define. Good for debate
Thing is, though, sometimes you'd get a mid-range bike that had a groupset slightly outside of your tolerances.
For example, my first real MTB (a 91/92 DB Apex, uncannily enough, I'm soon to acquire one...) was a mid-range bike. Cost, new, somewhere between 500 and 600 pounds. Had a steel frame, double butted. Groupset was Deore DX throughout.
Now that was a mid-range bike - there were two models above it (Axis and Axis Team), yet by merit of it being DX equipped, is outside of the mid-range BOTM criteria.
That's just an example, I'm not saying this particular case matters, I'm just saying we know mid-range when we see it, it tended to cost around (loosely) the £500 mark, if it was steel had a double butted frame, and a reasonably good groupset on. Some mid-range bikes had slightly up-market groupsets on, and some high-end bikes (I'm defining that on the spec of the frame) had mid-range groupsets on.
Bike mags new fairly well, when they were group testing mid-range bikes, the bikes they'd line up alongside each other - and they did it all by purchase price (that was within a certain range). People on here (myself included) can probably no doubt tell roughly how much bikes were, or where they fitted in the range (for the bikes people individually know about) - after all, somebody new posts a bike in the Reader's bike forum, and before long, you'll get several peole commenting on the likely year, spec, originality of kit, and price of it.