This is a difficult one.
We play to have fun. But of the 'fun' of any game whose goal it is to accumulate treasure is to grow richer. The richer we get the more powerful we become. And that's good. We receive our reward for succeeding and that feels right.
But as we all know by now, "With Great Power comes Great Ability to kick the %&*$ out of everyone"
As a Wizard and his warband become stronger the gap between him and his foes become wider and each exploration into Frostgrave can potentially compound the problem.
One solution is to throw more dangers at the more powerful Wizard as he enters Frostgrave. I think this is an excellent scenario tool. But for me at least I don't think its a ideal solution as it just doesn't feel fair that the most powerful Wizard all of a sudden becomes a magnet for natural (and supernatural) disasters.
Giving greater rewards of XP's to the Lesser Wizard also seems an excellent idea but again for me whether I'm the most powerful or the weakest Wizard this just doesn't inspire my competitive nature

Almost seems like every solution involves the victor to be punished or enter the battlefield with one hand (or wand) tied behind his back!
The problem here is that riches won in the game are solely represented in the accumulation of more powerful forces on the battlefield.
In reality and in history that is simply not the case. Riches equated to titles, property, privileges, popularity at court etc..
Each Wizard or "client" could serve a Patron, similar to the Patronage system in ancient Rome. The Patron would be the protector, sponsor, and benefactor of the Wizard;
The Patron should be some suitable archetype. Perhaps a powerful warmongering monarch, a Religious or Magical Institution. This Patron must receive the treasure collected but rewards the Wizard in turn. The rewards or resources returned to the Wizard could be randomly determined and may not actually reflect the total amount retrieved in their explorations. Moreover the unspent balance accumulates and at certain levels is actually used to express the Wizard's advancement in different categories.
The advancement within these categories are considered the real goal of their experience and riches.
So the Wizard's Patron confers titles, property, privileges, position at court, pleasure, honour, learning, knowledge, or harmony of the cosmos, the glorification and exaltation of their deities, or some greater cause suitable to the back story of the Wizard's school.
I think it might be interesting to devise or develop some detail to these a patronage rewards and how possessing greater wealth, power, or prestige could be represented.
Perhaps really cool magical items could only be conferred by Patrons as rewards for the exploits of the Wizard.
As the protector of their client or Wizard perhaps a warband that has been weakened can be strengthed by the Patron but only at the cost of lowering the prestige or reputation of the Wizard.
Anyway, I think as a campaign tool I'd be interested in developing this further.