A PC ram expansion board with 64k cost $350 in 1983 and upgrading it to 512k probably cost another $1,000. ($1,000 and $3,300 or $4,000+ for 512kB in today's bucks)
A significant portion of the cost of the Lisa (and later Macintosh and Amiga and everything else) was DRAM.
But RAM prices were falling rapidly and three years later when the Macintosh Plus was released with 1MB standard, page 57 of Macworld's January 1986 issue lists a 1 megabyte AST RamStak expansion board for the Mac XL (Lisa) for $829 ($2,400).
Even the Amiga 1000, remembered today as a revolutionary multitasking powerhouse, shipped with 256k standard in late 1985 and the 256k expansion that BARELY (fight me, I was there) enabled multitasking with 512k of RAM in total retailed for $200 ($600) bumping the price up to $1,500 ($4,500).
Cost was probably the most important thing to focus on, to spur adoption. Regressing to 128k though? That was garbage.