I had a boatload of souls saved up since the last Sac event, so I was ok going mad burning through them. I popped the 10 million once, and got a bunch more from achievements. Bought a couple soul talismans, and still have 14 million. It'll probably just build up until the next time to level up a couple talisman sets, or maybe until the next time you can get skill shards for gold. I'm just not sure what I want to shard next...