I don't have the book in front of me, so I'll let the Mongeese or someone who does respond to where it might be found (if it's there). It's probably down to GM fiat based on the weapon. If it is a small(ish) weapon that is not manufactured (balanced) to be thrown, the penalty would be minor (say, -10%). The larger the weapon, the stiffer the penalty, and some things just aren't meant to be thrown period. Throwing a great sword at someone would incur a very stiff penalty (if even allowed) and would probably be knocked down a step or two in damage, simply because it won't strike effectively but still could hurt if it actually hit (even if only by the flat of the blade). Because of the size differences in weapons, etc, my guess is the penalty isn't specified.