Paul: I'm also reminded of the age-old advice that "If it ain't broke, don't fix it."
And by what you've added to your original post I'm starting to think that your own site doesn't in fact comprise 100,000 pages. Rather, it serves as a kind of host to all the free pages, each of which has a specific domain (even if it is something in the way of a yourdomain/member URL) and links back to your home page.
Thus, it sounds as though you've created the likes of a directory site or a massive collection of info pages. If so, you can still turn the 100,000 list into a range of categories and sub-categories, can't you? This would retain the individual page URLs instead of changing them to, as it were, a set of remote domains. In fact, you could simply adopt my idea of a mysite.com/Mailings.html summary page that allows each newsletter to have/retain its own mysite.com/Topic.html URL.
Whatever the case, though, I'm now quite willing to guess that, from an
SEO/SEM viewpoint, you're wiser to retain all the pages as they are while, at the same time, restructuring things so there isn't so much of a "one whole pile" feel to everything.
Yes? No?
Duncan