In Das Schwarze Auge (TDE) Version 4.1, when I buy a talent for a single spell, I can improve the spell ability at a lower cost. Does this hold universally (i.e. during advancement) or only during character creation?
What you mean is "Begabung" I suppose? A "Begabung" (gifted in, talented in) holds universally, at any point and for all intents and purposes of improving your abilities.
In general, the only things in TDE that apply only at character creation are
- Academic Training (Akademische Ausbildung): Only during character creation, some skills only are 3/4 of the cost, only up to level 10. Which ones depends, in your case (mage) its languages, knowledge skills and some SF iirc.
- Veteran / Broad education: Those also have specific rules for the points you gain through them, one is that you have to spend them at character creation iirc.
All other benefits, where it is not explicitly stated to only be relevant at character creation, apply at all times - this includes "Good Memory"/"Eidetic Memory", "Merkmalskenntnis", "Gifted", "House Spell", and the use of teachers and books. Since they all freely stack, you can imagine the terrible impact this has on balance...