So your base fighter doesn't care about this sword as much, but anybody in the Swordmage, Paladin, et. al classes does. Basically, anybody who is martial with magical tones to it winds up with numerous powers that have typed damage on Weapon. I could also imagine an Eladrin Wizard using the sword as an implement, and asking for all of his powers to ignore resistance with it.
For the "feel" you're looking for, you definitely want to hit insubstantial for full damage. I was wondering about "ignore resistance equal to one-half the user's level" as an interesting balance point - slicing through only resist 1 at 2nd level, to a maximum of resist 15 at 30th level, which won't terribly nerf the most resistant of creatures. Ignore seems to be the right verb for the power to make it clear that it isn't reducing the resistance permanently.
Balance that against a +6 longsword which doesn't ignore resistance - hypothetically, that +6 should hit about 50% of the time, for, say, 2d8+14+(6d6-crit). Vs. a resist 20 creature, we're doing something like 23 points of damage on average, so we average 3 points per hit. We also average 21 points every 20 rolls for our crit, so let's call that 1 point per swing. That gives us an average of 10 points in four rounds of at-will attacks.
Our +1 longsword would hit 25% of the time, for, say, 2d8+9+(1d6-crit) damage, but ignores 15 points of resistance. The creature resists 5, leaving us doing 9 points of damage on average. Our crit is 3.5 points every 20 rolls, so in four rounds of at-will attacks, we average 9.7 points.
For my money, that's good enough - and that's not considering that our +6 may not be doing the typed damage our opponent resists, or should have its own powers for additional effects.
The place I'm wondering if you're on the wrong track is, why are you worrying about Level and Cost at all?
Give the weapon something like:
Property: this weapon is unique. The weapon's powers cannot be created, duplicated, enhanced, reduced, transferred, or disenchanted by any Ritual.
That ensures uniqueness, and gets you out of caring about what it might cost to level it up or down. It can't be bought. When you deal it to the party, its basically the highest-level magic item in the current level parcel. If they sell it, they can find somebody who is willing to buy it at, say, the equivalent of (current level minus 5).
