So I have since discovered that the skillpoint cost to increase each characters stats seems completely arbitrary, and cannot be derived from their base stats or their stat growth. For example, cirno, despite being slower than chen, remi, marisa, etc. Pays less skillpoints to raise her speed level than the faster characters. This is not a weird exception, I have seen numerous wonky examples. for example, 3 of the plus disk characters have the exact same base stat and growth for mag, (or was it atk?), two of them pay the same cost for skillpoints to increase that stat, another character seems to have to pay roughly 10% more for seemingly no reason however.
Has anybody found some kinda info page regarding how skillpoint costs are calculated or whatever? If not, don't worry about it, it'll just be nice to know if someone found one already though.