Just found out something about Dota Underlords

You can actually earn 1 extra gold for every 10 gold you have before the battle resolves. The max is 5 extra gold for more than 50 gold unspent. I find the best time to start saving 10 gold and accumulate savings / interests is from about level 8 onwards, usually level 12-13 when it begins. Now the game is so much more interesting and easier after finding it out.