Not necessarily. In most cases the most expensive part would be the precursor. The overall cost to forge it into the Legendary weapon, regardless of which one it is, would be about the same. Well, according to the API, it costs roughly 900 gold to forge the Moot with the crafted precursor. I don't know how buying the precursor factors into that cost (since you can only craft one precursor) but almost 2000 gold to craft two of the Moot sounds a lot more costly than crafting a second legendary that you already own.