I'm mostly with Swicord here, he kinda beat me to the punch. I'm gonna use "should" here, could have used "could" or "maybe" instead.
regular magic items should be easy to medium hard to produce but would have obvious level/power caps. Fighting would destroy an X% of them so as not to have everyone have several of them. Part of the remaining percentage could be captured intact and used or "deconstructed" for a percentage (not too high) of the original construction cost. Part should be damaged - giving even less in deconstruction or have a %age of the original cost for repairing them. Part of them shoud be "lost" (see bottom)
minor artifacts should be far more resistent to destruction, but might have a still significant chance of being damaged. Or maybe minor artifact should be "attuned" to the new channeler before they allow themselves to be used. Attuning would ofcourse require mana/time. This prevents them from being instantly turned on their creators and excessive "artifact hunting". Level caps are off. can be very powerfull. Are also quite expensive to construct (you might even have them cost a certain amount of upkeep - the channeler has to keep feeding them -> prevents overproduction). Occasionally one should be "lost". (see bottom)
Major Artifacts - not player manufacturable - serious quests to even get them - remnants from before the last great war - destroying them is not viable (otherwise they would hev been destroyed in the cataclysm) - at least untill very late game or with at least as serious a quest as for getting them. Extreme powerfull. Upon finding or capture needs to be attuned - very willfull things, so heaps of mana and/or time required. For a cost these items could be "hardened" - with that increasing the cost/time for others to attune them (giving you time to win them back or prepare for them to be used against you). WILL cost channeler from his pool to have around, perhaps even more when used. Only very rarely or not at all one should be lost, people tend to keep track of things like these.
Stuff going into limbo is weird. If one gets killed he drops his stuff. In order to prevent getting killed he's gonna go all out with those items, not spend energy in removing the items that he needs to fight from his posession. Besides...one of the "good" feelings in games like this is when you get to loot something good of the body of a vanguished foe.
As to the "lost" ones: They can be a continuing source of "minor" quests - just find the bloody thing. Should take some turns for the "legend" to occur. Might allso be a continiung source of "medium" quests....something found it after its been lost...and is not gonna give it up without a fight.
just my 2 cnt