I wrote 200 pages in a good sized notebook in my tiny, illegible handwriting and I now have the task of typing it up. I am slightly terrified that I'll do it and find that I have only half as many. Sure, I know the number of pages is not the most important thing, but I would really like to know how many pages I can expect.
Count how many words are on a page or two. It's alright to guess, but try to get it about right. Multiply that by 200. Now you have your wordcount estimate, which is the standard way to talk about a book's length. Contemporary novels tend to go about 80-100k, fantasy gets to be a bit longer, while YA fiction tends to cap at 75k.
If you have 200 words on every hand-written page, you'll have a half-novel of 40,000 words. I have a novella of 40k that measures 137 pages on a 6"x9" paperback. (292 words per page.)
Most fiction paperback books average about 250-350 words per page. The Hobbit is about 95,000 words. My paperback copy of The Hobbit is 303 pages, so with a bit of math we find it clocks 313.5 words a page. You can also get a feel for how type, margins, and dimensions change the page count by googling some of the well known titles on your shelf.
The way computer programs count words (and the way we should, also) is that every 4 characters = "1 word". For a hand-written document, if you don't want to literally count out every 4-character bunch on every page, do one or two pages, and then use that as an 'average' word count per page.