Pygmy Necklace

An extra minion is a flat gain in output that keeps applying while you do everything else, and it costs nothing to maintain once summoned. Pre-Hardmode gives you very little else you can slot for damage, so even a melee or ranged player who keeps one cheap summon out ahead of them gets more from this than from another movement trinket.

The Witch Doctor only trades after dark, so fold the purchase into a night you were already spending in town instead of making a special trip. The price is steep for a player who has not seen Hardmode yet, and clearing out the junk stacking up in your chests is usually the fastest way to cover it.