Spirit Orb boosts the rate of EXP gain by a percentage. Because each level for a unit translates to higher damage and HP, this is always a good upgrade to consider for a new unit. For units at the soft maximum level, this upgrade is useless because they cannot gain any more EXP. This upgrade is particularly useful on the Nova Turret because of the Galaxy Turret upgrade at rank 7.
|Level||Upgrade Cost||+X% EXP gain|