We know that already. ujhbn's formula is one-half of the actual numbers.
HP, BRV: 10% of EXP is returned as HP/BRV (100,000 EXP = 10,000 HP/BRV)
EX: 1% of EXP is returned as EX on the 1000-point Western scale (100,000 EXP = full EX bar)
Assist: 3% of EXP is returned as Assist on the 1000-point scale (33,334 EXP = full Assist gauge)
I posted some more info on assist translated from the Japanese wiki in the Battle Mechanics discussion thread.
I haven't worked on EXP bonuses from final EX bursts, which are irrelevant to competitive play but very useful for single-player Dissidia.