Does the cost change trigger on when-gain like Watchtower etc., or does it happen before that?
If you could order it with other when-gain stuff, you could gain a Salvager, play it with Innovation trashing a Wayfarer from your hand (for $6 let's say), and then apply the cost change ($4).
On the one hand the cost change does happen when (after) you gain the card, which is also when Watchtower, Innovation etc. trigger. But on the other hand, Wayfarer doesn't say "when you gain a card", it simply says that it "has the same cost as the last other gained card". So I assume the cost changes before when-gain stuff.
I would also expect Peddler to work the same way, changing its cost before Arena (start of Buy phase) triggers. (This could matter of you discard a Village Green with Arena, playing it using for instance Way of the Butterfly to return a Peddler Way of the Mouse to do something with a Peddler.)