Silver weapons are clearly a thing, as many monster stat blocks claim they can be hit with silver weapons. But I don’t see how much they cost.
So, how much does a silver weapon cost?
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A quick search of the Player's Handbook, Dungeon Master's Guide, and Monster Manual for Advanced Dungeons & Dragons probably led you to the same conclusion: beyond the entry for arrow, silver, single that costs 5 g.p. (Player's Handbook 35) and the Items On A Robe Of Useful Items table entry for dagger, silver (Dungeon Master's Guide 227) there appears to be in the three main books no other silver weapons.
In other words, the DM must make up his own material or use material from a source beyond the three main books.
Silver weapons are discussed in this 2016 blog post that traces the evolution of the inclusion of silver weapons from original Dungeons & Dragons through Dungeons & Dragons 3.5. It reaches the same conclusion, too.