I suggest figuring up how much the total amount of loot that the bandit may have had in his lair at the time, using the Dungeon Master's Guide treasure charts and the Monstrous Manual treasure notations.
Let's assume that they're all considered Bandit/Brigand types except the spell caster which would be considered a Wizard type.
Each Bandit/Brigand uses treasure types J,M,N,Q and the Wizard uses L,N,Q. We'll assume the leader is at least one to two levels in equivalent above his henchmen, so maybe count him as having an extra "roll" on the charts so he'd be considered 2x for his types of J,M,N,Q.
J types gives 3-24 (3d8) copper pieces, L types give 2-12 (2d6) platinum/electrum pieces, M types give 2-8 (2d4) gold pieces, N types give 1-6 (1d6) platinum/electrum pieces, and Q types give 1-4 (1d4) gems.
I'm going to go purely on thought.. so.. let's say he has 10 henchmen, excluding the spellcaster. The maximum possible loot would be 264 copper pieces, 88 gold pieces, 84 platinum/electrum pieces, and 48 gems. We'll assume each gem is worth 275 gold on average.
In my personal experience and opinion, the total bounty should never exceed what they would receive in total from the loot. Given they could potentially receive quite a haul (if you allow - I suggest rolling how much loot is in the lair, in case they face the bandits there), I'd feel 300-400 gold each sounds pretty fair. Of course, if they faced the brigands in their own territory and won, they'd be well-deserving of any loot found (if they feel inclined to keep it) and the rewards.
There are a few ways to go about the lair treasure bit, though, as... let's say, what if the local authority wants all stolen goods and such returned? Or what if the PCs think that the authorities want the stolen items returned, so they do so and are given a bonus (both in coin and experience) as a result? It's all up to you, of course. :3
What I'm doing, of course, is merely going by the guidelines of the books and assuming numbers of henchmen and the like. Make the rolls yourself to determine how much they might get off of the ill-doers, and figure up from there on whether it would be properly balanced for your campaign or not. If you think it's a little much, reduce it slightly until it looks like a comfortable number (but don't reduce it below the minimum, of course!).
Just my thoughts on this. Hope it helps!