To answer the question, you need the Hardness of copper, and the HP per inch. Apply damage normally, and any "destroyed" by the fireball was melted.
Well, I agree with the approach, but not necessarily with the details.
I also feel that if you are going for realism, the very basic rules for interacting with objects as presented in the SRD/RAW are not going to be adequate for even basic verisimilitude. The fundamental problems you are going to encounter are:
1) Lots of things that destroy tissue have very little effect on objects. So for example, a spear thrust might well go right through a creature, but probably has insufficient energy to do much damage to a hard object. Where as, the crushing blow of a maul will at least be as effective on stone as it would on soft tissue (albeit that stone probably has more hit points by volume). In general, I tend to half all damage against an object if the tool/weapon being employed is not designed to perform well versus the material of the object. So attempting to chop down a door with an axe works quite well, but is very slow going with a rapier - even if they both do 1d6 base damage. If you take the mechanical approach but ignore this, it won't be hard for most mid-level PC parties to bore through stone walls with their fists without too much trouble.
2) Similarly, attempting to attack an object with an item not designed for the purpose potentially does damage to the object itself. This is obvious on some level. If you try to break a door with a glass vial, the vial should smash. That however is not formalized by the rules, resulting in problems. The person attacking the door with the rapier if they apply too much force may well damage the rapier, while the axe wielder barring some sort of fumble ought to be fine (at least for game purposes, eventually of course any tool will dull or break from regular use, but the durations involved are sufficiently long as to be ignorable). And of course, attacking hard objects with your fists is probably a bad idea.
3) Finally, the rules give targets a static hardness and hit points that are linear with thickness. The problem with this is that realistically, hardness also linearly increases with thickness up to some quite high upper limit. That's because energy is absorbed and distributed more broadly the thicker the object, preventing flexing and deformation that might otherwise damage a portion of the object. A blow against a thin stone panel might smash it if it has sufficient force, but the same blow against a very thick boulder would normally do absolutely no damage at all. It probably won't even chip it. This is extremely important, because if you ignore the problem of thicker and thicker objects having increasing hardness, you get really dysfunctional results where objects that ought to be enduring and virtually indestructible without special tools, can easily be carved through in mere minutes without much effort simply by stubborn application of force. In the case of a thick wall of copper versus heat damage, copper will readily conduct the heat very quickly throughout the metal minimizing the actual effect of the heat on the object. The same amount of heat that would produce melting of a 3" thick copper panel might not even warm one that was 3 foot thick noticeably.
So there's no sudden, disproportionate melting of soft metals just because the spell name is Fireball.
But aside from the over simplified system employed by 3.X for interacting with objects, the mechanical approach is pretty much exactly what you should be employing IMO when adjudicating any damage to objects. I also agree that any exceptions to normal behavior ought to be clearly specified by the rules.
Personally, I'd rate copper close to Alchemical Silver, but harder than wood. Maybe a 7 Hardness and 10 per inch. Silver I'd rate a bit softer, and Gold softer still. Softer than wood.
I would rate copper as much more durable than you suggest here, with a hardness of 8 + 1/additional inch to a maximum of say 15 and about 20 hit points per inch. It would also in my game take half damage from any attack not specifically meant to effect metals, which in this case would mean probably anything but acid attacks, sonic attacks, and specialized metal boring tools. Faced with a 20ft thick copper wall, fireball would be virtually useless. Indeed, it would most likely take special preparation and considerable time to penetrate it. And anyone capable of constructing a 20ft thick copper stopper, probably also took the time to apply magical hardness spells and magical mending spells.
