Good News, Adventurers!
As of right now, we've removed all gold costs from battles.
You can read more about it on our official forums here:
http://community.gemsofwar.com/t/good-news-removal-of-gold-costs-for-battles/2950
We understand there's lots of stuff to spend gold on now, and the last thing we want to do is make new players feel unwelcome, so enjoy as many Quests, Challenges, Invasions & Defences as you like... all for free!