It would appear as though the amount of ultimate charge is granted based on the max health of the target when the kill is registered by the death-zone at the bottom of the pit.

Recently, I was playing Pharah on Nepal:Sanctum (the one with the death pit taking up half the indoor space), with ~11% ultimate charge. The enemy team was comprised of 6 Winstons. After dealing a moderate amount of damage to one of them with rockets (charging to ~26% ultimate), I managed to hit 4 of them with a concussive blast.

One managed to jump back up, but 3 unlucky monk-uhh, scientists fell to their doom. This charged my ultimate to ~87% (I was still firing rockets at this time, bear in mind). This indicates 20% per Winston killed, with 500 health per Winston.

I will perform some further testing later and update my answer accordingly with solid evidence and .gifs to support this case.

It would appear as though the amount of ultimate charge is granted based on the current health of the target when the kill is registered by the death-zone at the bottom of the pit.

Test case 1: Winston (500HP) Environmental

Test case 2: Winston (~240HP) Damage+Environmental

Test case 3: Winston (500HP) Full Damage

Test case 4: Winston Primal Rage (1000HP) Environmental

Test case 5: Tracer (150HP) Environmental

Test case 6: Tracer (120HP) Punch+Suicide

We can see from these clips that, negating the over time ultimate gain, the amount of charge gained is consistent with the total damage dealt, not including any measure of overkill.

For reference: For every 15 points of damage dealt by or as a result of Pharah's actions, 1% ultimate charge is granted. This means 33.3% charge for Winston, 10% for Tracer (consistent with what we see above).