From observations in an Asia Pac server:

Total damage for an entire realm's banish is now limited to 100m coins and 40m credits total, shared amongst all the realm's players.

Yes, it's 40m, not 20m like was said in the patch notes.

Rep gained is equal to the percentage of what you killed of the boss's hp.

Currently, this only affects 1.25b hp banishes and higher, since you'll never reach the limits anyway with a 250m hp banish and lower.

Calculations are an even percentage.

For the 1.25b HP boss and higher:

Coins gain = % of the boss's hp killed x 1M

Credits gained = % of the boss's hp killed x 400K