You're right, in Europe the Amstrad was less expensive than the Commodore and hence far more popular. The CPC 6128 was probably at this time the less expensive computer to run Turbo Pascal.
The Apple II has an optional Z80 card, but the total price was not for everyone, at least in Europe.
I don't remember the 6128 being cheaper than the C128, and the prices I've found online doesn't seem to support that either. I'm sure it might have been the case in some markets, or some configuration (since the C128 could be bought without a hard drive and without a monitor) but it seems far from universal.
Amstrad also sold far less than C128. The whole Amstrad CPC line sold about 3 million vs 5 million C128's. I don't know the breakdown between the Amstrad CPC models, but I'd bet a substantial percentage of that was the 464.
I'm sure the Amstrads were more common for CP/M, though. Of the few people I knew with C128's, most used it as a glorified C64, and nobody used the CP/M mode.
(Incidentally you could also get CP/M support for C64 through a cartridge that housed a Z80...)
The floppy disc reader was required for CP/M (and a 80 column monitor), so the comparison is with the C128 + 1571 or with the C128D.
If in 1985 the C128 had been cheaper than the 6128, I'd have bought the Commodore instead.
I rememeber the CP/M floppy ... I booted it once or twice but did not know what to do with it, so I ignored it. I usually booted my C128 in C64 mode to play games.
If you had a disk drive, that is. Many if not most MSX machines were typically used as game consoles running games from cartridges.
On CPC6128 the disk drive was standard (and yes, it was that strange 3" variant, not using the 3.5" disks that would later be known as the "3D model of the save icon".)
The Apple II has an optional Z80 card, but the total price was not for everyone, at least in Europe.