I'd switch Dungeon's top to +1 card, +2 Actions, +1$, discard a card.
Why? This should be too weak in games without Attacks (Bazaar). Maybe okay for $4. But I'd prefer something simple, maybe a $4 Woodcutter, or, as I said, a $5 Peddler?
It could also be a Treasure worth $2 that gives you Spoils per Attack cards in play for $5. (Edit: No - this is Silver in non-attack games)
The idea of Spoilage is, you want to play it when there are only 3 or less Spoils in the pile.
But if it's the only Spoils-giving card, then you have to depend on others to play it first, right? And regardless, you won't have much control over how many are in the pile when you get it into your hand.
I thought it might add some tactical depth to the decision whether to play Spoils or not. But maybe that isn't resonable.
Also, I really wanted an Attack that has something to do with Spoils (for the theme).
I thought of a friendly version of Spoilage first. Something like that:
SPOILAGE (Action) $5
Gain 3 Spoils from the Spoils pile.
Each other player may gain a Spoils from the Spoils pile. If he does, he gains a Copper.
My very first version was something very crazy - distributing all the Spoils to all players (with less Spoils available in 2-3player games).
However, I do still think that even though Duration card rules technically allow for this sort of thing, custom Duration cards would be much better off following the pattern of every other real Duration card and simply having a this-turn and a next-turn effect. This card could be reworked as a reaction instead.
The rules work for it, so why not? It might feel strange for new players, but I think they'd get used to it quickly.
A reaction would work as well, but I prefer the Duration version; I suppose it adds more tension.