Say I have a 100 page manuscript in Courier, 11pt, double-spaced. How many paperback pages is this roughly equivalent to? Is there a better font/spacing to use?
The standard font that I've seen in my experience has been Times New Roman 12pt with 1.5 spacing.
Honestly, there is no way to calculate how many pages your book will be until the publisher does all of their unique formatting and prints the book. Books are printed in all different sizes - some large, some small. Publishers also use different margin sizes- some like 1 inch all around, others like 1 inch on the sides and .5 in the headers. It varies from publisher to publisher. Some publishers also use different fonts.
Even Fox Cutter's estimate is 250 words per page isn't useful. If you have a lot of dialog, you may have pages with a lot less than 250 words on them. If you're a wordy writer, you may have pages with a lot more than that. Even the complexity of the words you use affects how many words are on a page. You 50,000 word manuscript may be half the page length of another authors 50,000 word manuscript.
The short answer: Don't worry about page length. Worry about word length. That doesn't change no matter how the book is formatted.
That's really hard to say, books are usually measured by word count and not page count. That being said you can usually assume about 250 words per a printed page. This means that a 75,000 word novel would be around 300 pages.
Even so, that's not a very good calculation. Different books and publishers have different layouts for their pages. For example, a YA novel might use a larger font size to increase readability leaving only 200 words on the page. On the other hand, a modern fantasy novel might have a much smaller font and tighter margins then you expect to cram 400 words on a page.
At one point in time My roommate collected the Shadownrun tie in novels. All the books are about the same page count, but the layout varies depending on the length of the story. In fact one of the books has a tremendous amount of padding all over the page to expand a much shorter story to the correct page count.
So, the long and short of it is that your layout and printed page count probably have very little to do with each other. I would recommend the word count trick from above to get a fairly close idea, but even that can be wildly inaccurate.
The software I use, Scrivener, offers "Project Statistics" which list words, characters, and estimates how many paperback pages (and printed pages) the manuscript will generate.
Here's a screen shot of the statistics for a project I'm currently working on:
You can see that this is a little less than 400 words per page in paperback. But don't forget that publishers employ different font sizes and leading depending on how long a book is in words. They'll pad out a short manuscript with large type and more leading, but lengthy manuscripts will correspondingly be compressed. I presume what I have here is an average of some kind.
Edit Here is the manual's explanation of how it calculates paperback pages:
So far Scrivener has been a Mac-only application A Windows version is supposedly going to be available this year. You can try out the beta here: http://www.literatureandlatte.com/scrivenerforwindows/
A publisher with a production department that knows what they're doing can estimate book length from a manuscript formatted in Courier 12-point, 1" margins, 25 lines per page.
Don't listen to writers on this; they think their word processor's word count means something to anybody but other writers. It doesn't. Don't listen to editors, either. They don't understand much better than writers. Listen to people who do book interior layout.