Short answer: a refillable-ink printer. Here's why it matters more than any other spec.
For printing proxy decks, the single spec that matters most is ink cost per page, not resolution or brand. A standard cartridge inkjet can cost $0.15–$0.30 per page in ink alone — for a 60-card deck at ~7 pages, that's $1–2 just in ink, and it adds up fast if you're testing multiple decks. A refillable-tank printer (Epson calls theirs "EcoTank," Canon calls theirs "MegaTank") cuts that to roughly a third of a cent per page once you've bought the ink bottles.

Wireless inkjet, ~$170, refillable tanks instead of cartridges. A full set of ink bottles (~$18) prints roughly 58 one-sided decks — that's about $0.30 in ink per 60-card deck. See the full cost breakdown for the exact math.
View on Amazon →Refillable ink tanks — the ink savings alone pay for the printer difference after a handful of decks.
At least 'Photo' or 'High Quality' print mode — most inkjets have this even on budget models.
Letter or A4 tray support to match your cardstock size (Proxidex supports both page sizes).
Wireless/USB doesn't matter for proxy quality — pick whichever fits your setup.

A full 5-bottle set (2 black + 1 each cyan/magenta/yellow) — enough for roughly 58 full 60-card decks printing fronts only. This is what actually makes proxy printing cheap; the printer is a one-time cost, the ink is what you'd otherwise pay per deck.
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