I use the ideas/rules below, happy to hear any criticism, alternatives, additions or improvements:
My categories (not quite tiers) of weapons
-Regular weapons
-Silvered weapons (or Mithril, or Truesilver, whatever the world has) these will successfully hit all but the most powerful undead, while regular weapons may do half or no dmg against (most) undead. Any weapon can be modified to a silvered weapon. But the smiths who can and do this work are one in three at best and it is not extremely costly but certainly not cheap. I was thinking about using a formula or % that you would add to the base cost of such a weapon, and that that is the cost for materials and labour to have it made into a silvered variant. For instance: weapon costs 10GP. Cost for silvered modification +40% = 14 GP. So you buy the weapon for 10GP, add the 14GP Mod, total cost: 24GP. The size of the weapon and amount of material needed could also factor into a formula. Perhaps if a Smith sells ready-made silvered weapons it could be a similar % / cost, not quite sure on this. Perhaps it should be higher or lower.
-Mastercrafted weapons, made by a Master craftsman/smith, these weapons have exceptional balance, design, finish, materials, hardening and craftmanship. As such they confer a (decidedly non-magical!) +1 to only attack or sometimes to attack and damage both. The cost is also at least twice the cost of a regular version. These weapons can not (or do half) damage undead or infernal creatures that are immune to weapons that are not silvered or immune to any but magical weapons.
-Magical weapons, these are generally Mastercrafted weapons that additionally have magic imbued upon them. Extremely rare and costly in my world, it is much more viable to find these in old, hidden, dangerous places than to find them for sale at all (those who have them almost never part with them), much less begin to be able to afford them. They always add a + to both attack and damage. They are effective against any undead and infernal creatures.
+1 are very rare 10.000 or less on the entire globe.
+2 Are legendary and extremely rare 200 to 1000 on the entire globe.
+3 Are mythical, 50 or less on the entire globe, only the greatest heroes in history ever had or have such weapons. Most will be sentient. Many will require the new wielder to attune to the weapons personality and to have a certain character and perhaps even perform a few tasks in order to successfully attune to the weapon and wield it with the full magical benefit.
Most of the additional functions and benefits or weapons in other editions are possible. For instance, an additional effectiveness against certain class of monsters or being able to shed a wan magical light and other small magical effects etc.
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Of course any special weapon should not be bland, I always have a picture and description when they find an unusual weapon.
The idea behind the divisions above is that weapons are very scalable in the sense of utility and power, rarity and cost. It gives the players opportunities to keep striving to unearth or obtain a better version through adventuring, far-flung travel and exploration. A variant of XP for Gold if you will: Magical weapons for “Dungeon” Delving.
It also makes sure that there will always be some enemies that are terrifying, if suddenly half or even the entire party can not do any or very limited damage against a creature at despite them all having regular or even Mastercrafted weapons, this can strike real terror in them, or force them to battle in a different way. It is also a way of keeping the power level and modifiers low while still making sure the players get useful and evocative goodies/upgrades.
Even DnD 5.0 has scaled it back, a +3 is also a very, very rare find according to those rules, long gone are the days of +4 or +5 weapons or almost any player having +2 from early on, I think that is a very good thing.