Okay, so.... funny exploit, the mastered weapons recycle for more than you paid for. So it is a small way to make money in the shop. Admittedly, the run does seem fun. GMO with the Apprentice would be to silly.
It might be more balanced if the % boost was based on some percentage of its base stat boost. So like, Laser Gun's 400% ranged gets 100% melee, 200 range, and 250 elemental, while Flamethrower gains 0.5% melee, 1% ranged, and 1.25% elemental. That way it's not ridiculously biased towards fast weapons.