[PvK]

Since nothing of any other size would ever use the proposed "Dwarven Axe", why would it be problematic to make it cost however many resources would be needed to differentiate it from the hammer?

Seems to me it could be even more clear than the difference between the MA Infantry of Ulm with Battleaxe or Maul - more resources for a slightly better weapon. You could also decide that Dwarven axes are more prestigious personal weapons than hammers and so have them cost +1 or +2 gold more to hire troops with those.