Mojo, it's tough to come up with numbers the way you're doing it, because even if you pick up two same-size paperback books, they could very well have different font sizes, slightly different fonts, different margins, and so forth.
I can tell you that a 250-page paperback, like, say, the length of an average mystery novel, is about 80,000 words (massive generalization), which is 320 pages (250 words to a page, and 80k / 250 = 320) in standard manuscript submission format -- Courier New 12, double-spaced, 1" margins all around.
Manuscript format has 250 words to a page. Something like Times New Roman 11, single-and-a-half spaced, with margins that are more like .8" or so, has around 350 or so words to a page -- although that's gonna vary even more.
A non-massive fantasy novel is likely in the 125k-word range -- that's the largest that most major publishers are willing to buy from a first-time writer these days, and they'll split a big novel up into smaller novels rather than publish a 300k-word novel by a newcomer. A 300k-word novel would be something like Jordan's earlier books. Not sure about Harry Potter. I suspect that Martin's books are also in the 200k-300k-word range somewhere, based on the number of pages and the size of the font. But he's a big ol' famous guy who can sell that kind of thing.
Right now, as totally general submission guidelines, the market is around:
75k-85k for a romance novel
80k-90k for a mystery
90k-100k for a thriller
100k-125k for SF or fantasy
At least, for new writers. An established person can get a much longer novel published, or scoot by with a shorter one.