What we get is: ~1642 coins per 1 xp:

purple rune gives 40 xp, sells for 10000

yellow rune gives 100 xp, sells for 200000

red rune gives 200 xp, sells for 1000000

That means yellows and reds you don't want (scorch, corrosion, wood) are worth selling, you should always synthesise other chakras.

Total exp needed to get a red rune from 1 to 11 - 67356 = around 112 mil coins spent to max a single red rune. Around 4 billion coins to max the feature with enough XP for full red rune set. Thought it would be more to be honest :D

edit: fixed mistake

Gives me about 10k power after 200k practice points obtained (over 600 mil spent into this feature).