I took slowed down videos with my phone because I had the feeling like Lunge was using too much mana. It seems like Lunge uses around 16 mana and not 8 as is mentioned. I removed the nodes Pilum Assault and Holy Incursion to test it without those and still the same problem persists.
On a sidenote, for Pilum Assault you have the text “This consumes Javelin’s mana cost”. You could add similar text for Holy Incursion since it consumes Smite’s mana cost.
I think the 8 is the base mana cost which is then modified by the distance you travel (“costs more mana the further away the target is”, from the skill description). If you use Lunge at point blank, it should just consume 8 mana.