Hello again. Another question again.
When does it become more profitable to buy coloured souls from Node and/or Mable, than just levelling up?
Well...
Colored souls give you +2 to one stat, or 20 max HP, or 10 max MP.
One colored soul is 5000 souls
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One black soul gives +1 to all stats + 10 max HP, and 5 max MP.
One black soul is 10000 souls (more profitable)
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One tainted soul gives +3 to all stats + 30 max HP + 15 max MP
One tainted soul is 30000 souls (same profit)
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One soul of the outer one gives + 30 to all stats + 300 max HP + 150 max MP
One souls of the outer ones is 300000 souls (same profit)
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One level up gives +1 to all stats + 50-100 HP~ and 1-2 max MP
One level up costs from 30 to 10030 on the levels 1-43, then in goes up to 12402 on the level 82, and drops to 10001 on the level 416.
And it will continue dropping.
500 lvl - 9838 for level up.
600 lvl - 9693.
800 lvl - 9490.
998 lvl - 9348.
But personally, since your level affects a reward for catching a blue fairy from the Chaos Dungeon, I suggest you to focus on your level and try to find those fairies.
If you get lucky, you can quickly level up without even fighting.
And of course, on the level 999, those blue fairies will give you 999000 souls, giving you ability to buy 3 outer souls and quickly grind stats.
Once you are strong enough, dominate the boss rush (left door in the Chaos Dungeon). In the end Mabel can give you 50 outer souls or a legendary artifact - Mystery of the Night Sky.
Though, I'd not go there without powerful skills and equipment.
Good luck!