I feel like cache and death cart (and contraband) already cover the big $ with penalty space, so heres some other ideas, centered around trashing/restitution.
Blood Money
$5 - Treasure
Worth $3
--
You cannot buy this unless you reveal an action card from your hand. When you buy this, trash an action card from your hand.
Blood Money
$4 - Treasure-Reaction
Worth $1
You may trash a card from your hand.
--
When a player trashes a card costing $3 or more, you may reveal this from your hand. If you do, place this on top of your deck and gain a silver, putting it into your hand.
Blood Money
$5 - Action-Attack
Each other player reveals the top 2 cards of his deck, trashes one of them costing from $3 to $6, and discards the rest. If any player trashes a card this way, gain a spoils, putting it into your hand.
For the first one, I don't know if having an action in hand (meaning you have to buy an action, draw it, then hit $5 without playing it) is a strong enough penalty, but its basic idea of requiring to trash actions somehow to get blood money. The second only barely self-synergizes, and is mainly something to look at in DA/swindler/saboteur games, following the restitution theme. The third, i think its kinda similar to rogue and some other DA cards, but i'll throw the idea out there in terms of what blood money could potentially be.