Well, you can barely have to buy any units if you play properly and sacrifice for troops while digging for chests like crazy. For example, in my no-loss game, before the last boss battles, I had around 3.4 million gold - and that's keeping in mind I never deprived myself from items I saw in the shops, liked them and bought them. I spent almost 2.4 mil on runes to augment my char skills properly.
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