[Hextech Crafting] Essence From Rerolls?

I don't really know how the additional Essence from rerolls part works. According to the League of Legends wiki:

Essence will also be awarded if the resulting content is worth less than what was used to reroll.

I've experienced that. I think I rerolled a 975 RP skin shard, a 750 RP skin shard, and a 750 RP skin shard and got Freljord Ashe (520) and 100 Cosmetic Essence. All those skins shards are worth more than the skin, so that makes sense that I got Cosmetic Essence. However, recently I did another reroll. I rerolled two 1350 RP skin shards and a 975 RP skin shard. I got Jade Fang Cassiopeia, a 975 RP skin. I did not get Cosmetic Essence. Two of the skin shards I rerolled were worth more than the skin. Do all three have to be worth more than the skin for me to get Essence?

Also, what determines the amount of Essence I get? If I reroll three Legendary Skin shards and get Assassin Master Yi (390), will I get 100 Cosmetic Essence, or what?

I haven't rerolled any champions, but do they work the same way as skins? Do I get additional Champion Essence if I reroll champion shards and get a lower valued champion?

If someone could help me understand the reroll system, I'd be deeply obliged. Thanks!