There is no RAW on this. However, the assumption is that shafted weapons are wood-shafted unless stated otherwise. Metal-shafted weapons are very unusual and would be much heavier. This is excluding the hafts of swords and daggers and stuff, which are mostly in hand and unlikely to be sundered. Anything with a shaft long enough that would be the primary sunder target is wood.
Unless you are dealing with special materials, in the real world metal shafts are fairly rare and sometimes when they do appear (e.g. Indian battle axes) they're hollow, which adds yet another layer of "how does that work?" to the damaging an object rules.
I'd let my PCs get something made out of all metal if they wanted. For a hand weapon (hand axe, battle axe) I'd probably call that masterwork but let it just be included in the masterwork cost; this is a thin or hollow metal haft (requiring good quality steel). You could even handwave it to be the same cost. A two-handed weapon like a spear, you might be able to get away with masterwork + mithril for the same weight but otherwise it'd have to be a lot heavier. I'd treat it as an oversized weapon.