Weapon levels, as with the perks, are static and are attached to that specific weapon. The damage and DPS values very mildly fluctuate.
Defining a "good" weapon can be a bit tricky.
If you are concerned only with damage values this should help you out. I would say find the second highest damage value for the weapon type you are looking for (I say second because most often first is some extremely rare piece of kit that is bounds above the rest). For a rough estimate, multiply it by 0.75. If your item has a higher damage value than the adjusted value you just calculated, consider it good.
Of course, many other weapons can be considered good due to unique attributes. Trepanners Requital/Grunmanns Bow arent the most powerful, but have a great unique perk that can cause chain reactions. Similarly Axe of the Dragon Hunter's damage isnt particularly stellar, but with an innate +28% crit chance and +5% damage for all enemies within 8m it certainly makes up for it.
What difficulty you play on also factors into the equation. I would say another rough estimate would be change the 0.75 value to 0.25 for routine, and 0.9 for perilous (consider 0.75 for threatening).
Hope this helps.