Small minions and jungle monsters have a 33% chance to drop a soul by default, though this changes throughout the fight in your favor if you happen to fall behind, and it never gets lower than 33%. Large monsters, minions and champions drop souls 100% of the time, though Nunu drops 2 (because of the yeti). If you can't physically walk to the souls, his lantern can pick them up as well, allowing you to farm souls safely from a distance.
The diminishing returns the more souls you have was dropped a long time ago to incentivize Thresh to continue harvesting souls into the mid and late game. Now they all give a static .75 AP and Armor. Lifted directly from the wiki:
- 64 souls (slightly over three and half per level) are needed to match Gnar's 64 armor, the lowest innate champion armor.
- 104 souls (almost six per level) are needed to match Maokai's 94 armor, the highest innate champion armor.
- 123 souls (almost seven per level) are needed to surpass Taric's passivelyShatter-enhanced 108.1 armor, the highest possible persistent armor without the use of items, runes, or masteries.
- 264 souls (almost fifteen per level) are needed to surpass Rammus's Defensive Ball Curl-enhanced 213.4 armor, the highest armor possible without the use of items, runes, or masteries.