The Amiga parts are expensive and they were not made to last past 10 years. So collecting Amiga systems is expensive. There are emulators for the Amiga that make it cheaper. Even a white box OS http://www.hd-zone.com/amithlon/ to run Amiga programs.
What is there available in NeXT emulation? I went with the Amiga not the NeXT because the Amiga cost less.
I was chuckling at the insane dedication of the Amiga folk, who have at this point built all-new Amiga-compatible 68k hardware far beyond what existed at the time, new generations of system software, modern repro parts for everything, an incredibly robust emulation ecosystem, secured distribution rights for or replaced almost all the commercial software that existed for the platform, etc.
I have an A500 in my collection, they're interesting machines, and fortunately not workstation-expensive.
There are several decent options for emulating NeXT stuff. Previous ( http://previous.alternative-system.com/ ) emulates Cubes and Stations quite well (and its 68k core is derived from Hatari, which is derived from UAE, bringing this full circle).
x86 OpenStep runs well in QEMU (with some careful VM configuration choices) or VirtualBox, there are several guides for setting it up.
The guys that created the vampire accelarator boards for Amiga now have a open source (ish) OS called Apollo OS. It has a hardware abstract layer which hopefully means there will be a decent x86 port.
What is there available in NeXT emulation? I went with the Amiga not the NeXT because the Amiga cost less.