Combat spell cost abruptly increases 150%?

[Qeveren]

I'm not sure if this is a bug or some other effect I'm not yet aware of, but I just noticed that my spell costs in combat have abruptly increased by 150% between one turn and the next. I was summoning Phantom Warriors at 10 mana, now they're 25? No idea what's happened there.

[Hoverdog]

Casting cost of spells in battle rises with the distance to your capital.

[OneFiercePuppy]

Casting cost of spells in battle rises with the distance to your capital.

Minor nitpick: that's distance to your nearest city. Or else casting would be slow, indeed, in all the rest of the planes, unless you used "Relocate Tower" regularly. Which would finally give us a reason to use it more than never

OK, yeah, it's a good tactic to avoid banishment if you don't have a return spell in storage, too. Much cheaper than casting return, as well.