The gold cost you seen on the Power Promote screen is actually the cost after the gold you earn from raiding is use to pay for the badges you need to craft. Using your example (and adding a couple of zeros so the numbers add up correctly):
The actual cost of crafting all the badges needed to Power Promote that hero is actually 60M, so after earning 50M for raiding, you still owe another 10M which comes out of your gold supply, lowering it from 1.1B to 1.09B. It’s actually a variant of the same question that’s been asked many times about the cost of Power Crafting, which was discussed in nitpicky detail here.
If you’re wondering how I can be so confident about that explanation, look at this Power Promote screencap:
If I’m having to spend over 14k stamina in order to promote Clawhauser to O4, then obviously I’m having to craft quite a few badges, yet the gold cost shows as zero. The only way that’s possible is if the game is showing the net cost after taking into account the gold I earn from raiding. QED.