Paperbacks can be anything from $5 to $20. It seems a very imprecise system of measure :|
I go by the price of my favorite meal from McDonald's, which is just under $5. Since the items are value meal, it's roughly the same just about everywhere I go.
Where I live and in most places I've spent time in, pretty much every book store I've ever been in, from indie stores, to good chains, to bad chains, to absolute shit chains, charge $7.99 for SF and Fantasy paperbacks. 
Of course, hardback and stuff from other genres is a different matter, but SF/Fantasy paperback prices are about as stable as manga prices (too damn high; used to be $7.99 per volume, which was reasonable, but are now apparently 15.99 per volume-scanlations it is, then).
Also, still got a stack of cash from reselling my texts from last semester, so I'm going to wait and jump on special sales for stuff I wouldn't normally buy (and to see if I can actually run the stuff I bought, in which case a bunch of new avenues are open).
Depends (mostly) on the size and age of the book in question.
I have in front of me three different sci-fi paperbacks: The Founding (Gaunt's Ghosts omnibus), A Mighty Fortress (by David Weber) and A King of Infinite Space (hardcore sci-fi that no-one, not even TV-Tropes, seems to know about).
The Founding: 767 pages, $15
A Mighty Fortress: 1130 pages, $8.99
A King of Infinite Space: 424 pages, $5.99 when new (got it for $1 at a used book sale)
You can kinda see why I wouldn't use that measuring system