"Trading in" Wizard spells

[darksoul]

I'm throwing my two cents in.
There is another alternative I've been considering. Cumulative spell knowledge.
The idea is that lower spells are used to understand higher level spells of a similar type. So, a fireball spell is an upgraded version of a firebolt for example.
Learning a fireball spell would be easier(less xp) if the wizard has learned firebolt. Once learned, the spell is never forgetten, just a subcategory of fireball.
The problem is that there maybe many spells that can't do this.

[Dawndeath]

Nevertheless, the underlying logic is sound. I quite like this idea. We could compose 'spell lists' - like those found in Rolemaster - of somewhat related spells. You can still buy any spell you want at the regular XP cost; but if you have the earlier spells in a particular spell list, you can buy the next spell in that list at a reduced cost, because you already understand the underlying concepts of that type of spell.

As you say, not all the spells could be related in that way, but I don't think they all have to be. It's an in-world explanation that makes sense of a potential game mechanic.