This is definitely a case of 'kindness comes back to you'! It's a reward because I did something good!
Literal
This [topic-は] definitely, kindness [topic-は] person [gen-の] sake not-[copula-ならず]! Good-thing did [because-から] reward [copula-なんだ] [emph-よ]!
情けは人のためならず is a famous proverb — literally 'kindness is not for others' sake,' meaning that kindness ultimately comes back to benefit the giver. Many Japanese misunderstand it as 'kindness isn't good for people' (i.e., coddling them), but the traditional reading is the opposite. ご褒美 'reward' uses the honorific ご prefix. The speaker gleefully connects a good deed to the reward they're receiving.