I was taking inventory of all my Spellements and discovered an inconsistency. When upgrading spells from Tier 2 to Tier 3, some spells cost 115 while others cost 105. There seems to be no pattern or reason for this. The vast majority are at 115. I personally don't have all my spells at tier 2, and I have some at tier 3 where I can't check, but here's a couple that I found at the incorrect 105...
1. Thunder Snake 2. Old One's Endgame
I'm guessing there are more, and community please feel free to comment with any others you find! It'd hopefully be easier for you the devs to go in and check these though.