Reduced mana costs? Or if you'd use item "-1 to all spells" you would still be able to use them.
Other than that: no. There is no synergy system in belzebub and there wont be one.
Higher mana costs? Thats weird, considering how the effect doesnt change. Well maybe for the telekinessis, i heard it can be used on monsters to stun them but idk, i never tried that. Also sometimes character level affects the mana cost. Overall id personally max all the spells for fun, just so i can say that i have them maxed
Hey, you were right, I thought the mana cost went up, but it actually went down when I added some skillpoints to a skill.
Now a bit stuck though at lvl 34, somehow the leveling goes much slower since this level. I'm now playing Nightmare lvl 13 but the exp gain isn't high enough.