I am aware from the Sage Advice that potent spellcasting adds damage to both the first and second enemy when using green-flame blade.
What I want to clarify is that this means that the second enemy takes my Wisdom modifier (I am playing an Arcana cleric) twice?
We are starting at level 11, so if this is the case then I would be dealing 5d8+18 for an average of 40 damage (I have a 20 Wis and a 16 Dex. I am using a rapier as my weapon) with a cantrip.
Is this high for a cantrip at this level or pretty average?
Best Answer
The damage stacks
The secondary target takes your Wisdom modifier twice, as you wrote it yourself.
Very good damage for a cantrip
Only Eldritch Blast can come close to the raw damage. Compared to weapon attacks, it is OK.
A Fighter with a Greatsword could inflict the same damage, but with a much better chance to hit, so a higher DPR.
Calculating full damage for the secondary target is misleading. Even if there is a second enemy in range, spreading damage is usually weaker than focusing it. For this reason I would calculate (as was accepted on the WotC forums while they existed) by halving the secondary damage.
DPR calculation
At level 11 you can expect to fight against CR11 monsters; they are expected to have AC 17.
Green-flame Blade + Potent Spellcasting
Hit chance: 55% (4+3 vs 17)
Damage: 31 \$=\left(8+4.5\times3+\frac{10+4.5\times2}{2}\right)\$
DPR: 17.05
Great weapon Fighter
Hit chance: 65% (4+5 vs 17)
Damage: 40 \$=(3 \times (8.33+5))\$
DPR: 26
Warlock with Agonizing Blast
Hit chance: 65% (4+5 vs 17)
Damage: 31.5 \$=(3 \times (5.5+5))\$
DPR: 20.48
I would highlight that the Warlock and the Figher can spread their damage among multiple targets if they so wish, but your Green Glame Blade has no such option.
Conclusion
It is very good damage from a Cleric without spending spell slots, but not spectacular in itself.