|Image from Wikipedia|
Some foes, such as lycanthropes and wights, are immune to standard weapons but vulnerable to silver, and most or all versions of D&D include silver versions of various items in the equipment list. OD&D prices silver arrows at 5 GP per arrow (compared to a quiver of 20 arrows for 10 GP), and Moldvay prices silver daggers at 30 GP (ten times as much as the standard dagger). Silver crosses are also available for 25 GP (the simple wooden variety is only 2 GP).
However, treasure hunting adventurers are not very price sensitive regarding mundane equipment, so increased cost does not have much effect other than during initial equipment buying. Thus, there should be some trade-off to using a silvered weapon other than just costing more initially. Otherwise, players will just outfit everyone with silver versions of everything, and then combattants will be assumed to always use silvered weapons, just in case. Most things that are unproblematically better are boring. So there should be some reason to not use silver weapons all the time.
A silvered weapon is not actually made of solid silver. Rather, it is an iron or steel implement that has silver bound to the blade in a process similar to gilding. Perhaps a ritual and some hedge magic or blessing is also required as part of the procedure. As it is used, the silver wears off. This process of wearing off is actually critical to the effective functioning of the silver weapon — you are essentially leaving traces of poison in the argyrophobic creature.
A “silver die” (d6) should be rolled along with every damage die. On a silver die roll of 1, the silvering process has worn off, and must be re-silvered. Needing to roll an extra die also draws attention to the use of a silver weapon, making it more of an explicit choice, and less of a default. This makes silver arrows potentially more cost-effective than most silvered melee weapons (though note you can’t effectively fight with a ranged weapon if you are in melee). Also, a miss with a silvered melee weapon will not potentially degrade the weapon, but an arrow that misses may be damaged or lost. So the value comparison is not direct.
Most metal weapons can be silvered. The cost (following Moldvay) is ten times the normal weapon, and takes a skilled smith one week to complete. Given the cost of silvering, it makes sense to only use silver weapons when they are likely to make a difference. This is in effect a form of melee ammunition.
Silvered plate armor is available too, at the same cost multiple. Argyrophobic foes will generally prioritize attacking characters that are not wearing silvered armor, and will usually take a penalty when attacking combattants armored in silver (though this varies based on the specific creature). Silvered armor will also wear out in a similar manner (a silver die should be rolled per attack that is landed on the wearer).