I have a 2009 27 inch iMac that I use as a spare monitor though I haven't needed to use it in over a year.
I have a 2010 27 inch iMac that I use as a TV set. I paid $100 for it many years ago. If you have an Apple Silicon Mac that includes a 27 inch 2k monitor and speakers for $100, I'm all ears.
I have a 2015 27 inch iMac that I run my office stuff on. i5, 1 TB HDD, 32 GB of RAM, 5k monitor, speakers, microphone, webcam, Apple keyboard and mouse for $200. If you have an Apple Silicon machine that comes with a 5k monitor, speakers, microphone, webcam and 32 GB RAM for $200, I'm all for it.
I have a 2014 MacBook Pro 15 on loan to a relative for work.
I have a 2015 MacBook Pro 15 that's my backup for my 2021 MacBook Pro 16. I sometimes loan it out to relatives if they need a Windows machine.
There's an iMac Pro for sale nearby for $900 with 32 GB of RAM and 1 TB SSD and I'm somewhat interested.
One thing of note is that I have a program that uses 800k RAM on Windows, 5 GB RAM on macOS Intel and 8 GB RAM on macOS Apple Silicon. It runs better on an old Windows box than it does on Apple Silicon but that's partially because it crashes and corrupts the profile on a regular basis. It usually takes over an hour to reinstall on macOS Apple Silicon.