Since the mission update, the prestige rewards have been the same. It's too unfair to complete missions on normal and still get rewarded the same prestige as someone who does it on elite. I'd say, balance this out.

Consider keeping the prestige the same for advanced, since this difficulty level was considered equal to the difficulty before the battle sprites update.
Then decrease the prestige of normal and buff the prestige from elite.

I can't really give a fair value. These rewards vary quite a bit, but here's an example.

Original Prestige Reward: 180 Prestige

New:
155 on Normal
180 on Advanced
205 on Elite

(I know that 25 prestige per difficulty could seem awkward, but it's just here for the idea.)

Also as a possible bonus: when you've done the missions on either normal and advanced, you could redo them on higher difficulties to gain the prestige that you could still achieve.
(If this'd count for rank missions as well, I'd say hand those who've already redone the missions on elite their respected prestige)