Fairly simple suggestion really:
Add the expected increase of gildar and mana next to the current gildar / mana at the top bar of the screen. Such as:
Gildar 549 (+12)
Mana 74 (-2)
Also format the number green / red for additional effect.
Seeing the expected increase right there lets us easily plan if we should cast that additional city spell that has maintenance without having to check if we are going to negative income by hovering over the gildar/mana.