I guess at some you could have some card saying things like "1 VP per each other victory card giving less than 3 VP in your deck", and then giving nothing would result in weird questions, while giving 0 is cristal clear. Assuming every Victory card gives some amount of VP (may be variable) is refreshing, as is assuming each Treasure gives some amount of coin when played.
Amalgam - Treasure - $3
If you played other Treasures this turn, this is worth the same as the minimum amount of coin given by a Treasure you have already played when it was played. Otherwise, this is worth $1.
I hate that I cannot say "In play" due to Counterfeit and buying Mint or Mandarin from the Black Market (I mean, it would be less clear if it says "In play", because it would raise questions about Treasures that were played but are not in play, since its at the same time referring to what happened when they were played).
So, it could usually be a Silver, but it can also be way better than that. Note that it would only copy the value and not other effects, so HoP or most of the Prosperity Treasures will nombo with it (except Hoard, which would be great).