Looking at System76's equivalent mini, it's $555 for the low end, which has the same RAM and a faster, newer processor. Going to 32 GB takes it all the way to $700, or half what Apple's charging. So yes, "extremely expensive" is what it seems like to me.
Especially given that it should be entirely unnecessary. Just let me do a cross-platform build!
Of course, if you want to test your software on Macs, you'll have to buy their hardware sooner or later, but even then you have to set up the build on the system instead of keeping it as pure test system.
If you do it for your day job, it's probably justified, but if you are just a hobbyist, then even hundreds of dollars are expensive. Especially if you live in a poorer area or are still a student.