Derren points out that PC's might try to abuse the rust monster by penning them up and feeding them old magical items to get the full value of residuum. Some people are trying to disprove this, but all I can say is dear God in heaven and his virgin mother, I hope he's right.
First of all, the only thing they can do with the residuum is rituals and crafting other magical items. I don't give a damn if the PC's have extra residuum for rituals, because I want to encourage more rituals in the game. I love rituals like "seek rumor" and "silence" and "tenser's floating disk" and all the other utility spells and find them a little too expensive to show up as often in the game as I'd like.
For crafting magical items, you can't craft items of a higher level than you are, and the exponential cost of magical items pretty much puts the kaibosh on using residiumm from a bunch of old items into a residuum for a new item. The only thing a rust monster allows you to do is reconstiute a magical item of the level of the item you've lost, which is deliberate so you don't wind up losing your favourite magic item forever (just for awhile).
However, if a PC tries to capture several rust monsters for breeding, it can make a good subplot for at least 3 levels of adventures in which a PC tries to find rust monsters, contain them in a stronghold, and try to breed them. The output of fresh rust monsters will be fairly slow (since you have to kill a rust monster to retrieve the residuum, thus reducing the herd) if one assumes a breeding pair can lay 1 egg a year, and rust monsters live about 20 years or so.
You could perhaps get a small advantage from using raise dead on a rust monster, as long as the magical item was worth more than 1/5 -500gp that you'd gain in residuum. You couldn't buy magical items for this purpose, because you'd lose 500gp above the cost of buying the residuum in the first place.
Really, getting 100% residuum on heroic tier just seems to be a delightful way to compensate the PC's for all the trouble they are going through. They'll cast more rituals, and they'll take more interest in something other than dungeon crawling and killing loot. It is all full of win!