You could upgrade the Amiga 500 to 1 meg, using the trapdoor expansion. This was a very popular configuration.
I had a SCSI and RAM expansion for my Amiga 500. It gave me 3 megs and a whopping 30 meg hard drive!
The Amiga was so far ahead of early DOS machines, it was pathetic. Of course, by the time 486, SVGA, etc. came along it was pretty long in the tooth...
An Amiga 500 was around $600. (edit: $700) A Sun 3 color workstation could easily be $20,000 or more.[1] In today's money that Sun would be more like $40,000.
Also, I don't believe you're correct about the memory being non-upgradable. A cursory Googling suggests at least 8MB could be added, maybe more.
[1]http://textfiles.com/bitsavers/pdf/sun/Sun_Price_List_Dec88....
Uh... the 68000 had, in fact, exactly 16MB of addressible space (no MMU and a 24 bit address bus).
And, I dunno, arguing about the benefits of a flat memory model on a device that shipped with an non-upgradable 512kb of memory is a little spun.
The 68k was a more forward-looking ISA for sure. In consumer computers of 1987, that didn't buy it much. Look to the '020 machines Sun and others were shipping for examples that make it more attractive.