OK here's a picture of some "M" dies and parts.
Left to right, there are two 'long' die bodies, a 'short' die body (I think), two RCBS handgun expander die bodies which can be used, two (upsidedown) long stems (one with expander and one without), two short stems (also upside down),and some expander plugs.
As you can see, the main components are the die bodies, stems, and plugs, with locknuts to hold them in position, of course.
Rather than have a complete set for every caliber, I've chosen to have just a few die bodies and instead have more stems and expanders (others not shown). The reason for this is that I have two presses that I might use, and I have set up the longest and shortest dies bodies for a specific press. You might be able to read "RCBS" and "Dillon" written on the die bodies, which are the two presses I use.
The die body in the middle, the 'short' Lyman, is an extra and can be adjusted enough that it will work for either rifle or pistol, depending on the stem length. So it gets used for experimenting.
I set up a stem/expander combo for a cartridge, and use an extra lock nut so that the adjustment isn't lost between the presses. So it's pretty quick to set up and the stem/expander can go in the box with the rest of the usual die set, and if the right die body is used, can be switched between presses without having to be re-adjusted.
Adding a new caliber then only requires a stem of the right length, an expander, and perhaps an extra lock nut.
Since I can make the expander buttons and stems pretty easily - basic lathe work - it costs me nearly nothing to add new calibers. Of course this isn't true for everyone, but still, just buying the stem, expander, and locknut 'should' be cheaper than an entire new set.
If you only have one press then this might be overkill. If you only have a few calibers to reload then you might just set up an expander in a die body for each caliber, a small expense. But once you get your first "M" die, I predict you'll be getting them for the rest of the calibers you reload for, whether for cast or jacketed.
It's truly one of thos products that you wonder how you ever did without it, once you use it.