I think it would be near that many pages, for many books around that size, at least. It depends in large part on how many sentences you fit on a page, after all.
Anyway, you're claiming a false dichotomy there. Standardizing on single spaces is an incredibly easy fix if you want to minimize space used up, and the space savings far outweigh the effort (regardless of the cost in readability). In fact, shrinking typefaces will damage readability more quickly than eliminating the second space between sentences -- so if you're going to shrink text, you might as well wipe out a bunch of spaces while you're at it.
It's silly to talk about whether shrinking margins would save more space than getting rid of "extra" spaces between sentences, when it's quite simple to get rid of the spaces whether you shrink the margins or not.
Anyway, you're claiming a false dichotomy there. Standardizing on single spaces is an incredibly easy fix if you want to minimize space used up, and the space savings far outweigh the effort (regardless of the cost in readability). In fact, shrinking typefaces will damage readability more quickly than eliminating the second space between sentences -- so if you're going to shrink text, you might as well wipe out a bunch of spaces while you're at it.
It's silly to talk about whether shrinking margins would save more space than getting rid of "extra" spaces between sentences, when it's quite simple to get rid of the spaces whether you shrink the margins or not.