A couple of my thoughts...
Dragon Abbey does not improve its Mana reward by Chapter. Game progress with increasing demands for Mana effectively means decreasing usefulness of Dragon Abbey.
Mana decay rate does not increase by Chapter.
Lava Egg, Chapter 18, 108.1 Mana/sq/hr Lava Egg, Chapter 21, 210.9 Mana/sq/hr, 195.1%
Watchful Winter Owl, Stage 10, Chapter 18, 109.4 Mana/sq/hr Watchful Winter Owl, Stage 10, Chapter 21, 213.3 Mana/sq/hr, 195.0%
Manta Ray, Chapter 18, 111.1 Mana/sq/hr Manta Ray, Chapter 21, 218.8 Mana/sq/hr, 196.9%
Somnial Pool, Chapter 18, 121.5 Mana/sq/hr Somnial Pool, Chapter 21, 236.1 Mana/sq/hr, 194.3%
Mana producing buildings increased their output efficiencies by nearly double from Chapter 18 to Chapter 21.
Mana requirements increased their request by slightly over double from Chapter 18 to Chapter 21.