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Living Forgotten Realms bans that silliness outright. It has a strong potential to break your economy if you don't keep careful watch over it.

But if you want a way to handle it? How about the local mages' guild being unhappy about someone else cornering the market in ritual materials? Sounds like a visit from the local goombahs to trash the store, kill the rust monster, and leave a message not to meddle in the affairs of wizards?

Also: captured monsters are an extra statblock and bookkeeping to bring to the table. Treat them like mounts, and you'll never see them mentioned again properly. It's rarely worth anyone's time, from a meta standpoint, to bother with capturing something other than to turn it over for the reward at the end of a plot hook.

My two copper.

 The most relevant fact of the situation is this - in order to produce residuum, the rust monsters have to be fed enchanted items. And you control how many enchanted items there are in your game, as well as how much monetary and other treasure the party gets from their adventures. If necessary, adjust the amount of non-magical treasure you're giving them to account for the extra amount of wealth that they're going to gain from harvesting residuum.

There's no game mechanic for training a rust monster, so even if the party decided to haul these things around in a cage and try to unleash them on their enemies, there's no guarantee that the rust monsters aren't going to turn around and attack the party again. And then the party has to worry about how to get the genie back in the bottle (or cage, in this instance), so to speak. At best, there may come a future moment in the story where the party dropping a pair of rust monsters into the mix may be an interesting and canny tactic or just the right solution to a problem, but it's extremely unlikely to happen more than once and the hassle of caring for and transporting the things around (even in a bag of holding) may not be worth the effort.

 Given that there's not going to be too many times the party will have a spare magic item to feed to their new "golden goose", it shouldn't represent much of a problem.

There's also this (assuming you're playing 4e):

Devour Metal • Recharge if the power misses
Attack: Melee 1 (one creature wearing or wielding a rusting item); +11 vs. Reflex
Hit: The rusting item is destroyed. If the item was magic, residuum worth the item's market value can be retrieved from the rust monster after the creature is slain.

"Although most humanoids regard rust monsters as a plague, the creatures do offer a boon to anyone who manages to slay them. A rust monster often consumes magic items, and as it devours the metals of an object, its body stores deposits of residuum created by the decomposition of the item."

To retrieve the residuum, the rust monster must be killed. That puts an end to the farming right there.

Nov 9, 2012 -- 12:05PM, Rood.Inverse wrote:

Living Forgotten Realms bans that silliness outright. It has a strong potential to break your economy if you don't keep careful watch over it.

The reason LFR outright bans it, is because unlike in a home campaign we have a lot less control on what the PCs find. Of course, as a DM you should realize that a rustmonster does a 1 on 1 conversion while the market does a 1 on 0.5 conversion. Hence it will skew your gold:xp ratio which has an impact on balance.

As for the natural course, that seems odd (although not more odd than a monster that eats metal). The rustmonster eats the stuff to get its energy from somewhere. I would assume it actually uses it to stay alive, so what ever excrement it has, it would be valueless. Obviously, you can change it, but realize that doing so will screw over the economics of your game (both the world and more importantly that of the xp:gp ratio of your group).

Note btw that the monster is going to have an impact beyond finances. As others pointed out, it is another set of stats at the table which can be confusing and a potential source of irritation when the pet is killed, when it is not "properly" leveled or when it starts targetting PCs' stuff (or worse: random NPC's stuff on the street). So you need to discuss with your group how you are going to deal with such issues.

Another potential problem is how it impacts certain challenges in an adventure. I remember an adventure in my 2e days with a wild mage who accidentally summoned a rust monster. At the time they were imprissoned and without equipment behind steel bars. It was a life saver, and they lured it to other areas to deal with the guards and the vault doors behind which their own equipment was hidden. It was not exactly how I planned it, but the players had fun even though it was a bit easier than fighting heavily armed opponents (or bluffing one way out). For me that happened only once since the monster was summoned and disappeared after x amount of time. If it is always present suddenly those iron portcullis, steel vault doors and steel bars become a lot easier to get past