agburanar

Rather than a hard cap - make a soul gem degrade when used. So using a full grand gets you a greater, and so on. That way the quest reward is still unique.

One other way to limit it without removing it - making the reduction effects stack as multiples instead of additions. So two 25% reductions gets you a 45% reduction, three gives you 59% reduction, four gives you 68% reduction, etc.