Trench
Action - Reaction - $5
At the start of your next buy phase, +1 buy and +$2 per unused buy.
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While this is in play, cards cost +$1.
If another player gains a card other than during a buy phase, you may play this card.
Is this fair at $5? Should it be an attack? Should it be a duration?
Leper
Action - Attack - $4
Gain a copper and a silver, putting them into your hand.
Each other player gains a curse. If they do, they gain a Leper.
Does this seem fair at $4?
Leper seems fine, except Curse, Copper, and Silver should all be capitalized. Trench has some clarity issues. Here's what I think you're trying to say:
Trench:
At the start of your buy phase, +1 Buy, and +$2 per unused buy you have.
While this is in play, all cards cost $1 more (this applies [after/before] cost-reducing effects).
When another player gains a card outside of his buy phase, you may play this from your hand.
Cost: $5
Action - Duration - Reaction - Attack
It should be a Duration and an Attack, but it only works as either if you do the reaction. I added the clause in parentheses because the order for applying cost-reducers vs cost increasers matters for things that cost $0, because if reduction happened first, they would cost $1 after this, but if increasing happened first, the cost would remain at $0 (I suggest increasing applying first).