Each signet is worth 18,651,741.33 coins. This calculation is made by subtracting the dragon crossbow and 100 ascension shards cost to make the ascension crossbow, while dividing the rest by six as there are six signets, so one could assume each signet is worth this much. When a signet is dropped, a message in the game box will say "You spot the Legio's signet on the floor!"
Signets are always tertiary drops. The drop rate for an ascension signet is 1/50.