[US] Newegg: ASUS AREZ Strix Radeon RX Vega 56 - Open Box - $219.99
Navi is right around the corner but the RX Vega 56 remains the best value GPU for Mac users. I used this same ASUS Strix RX Vega 56 in the XG Station Pro before and it's an excellent combo [build guide]. It provided great performance boost and ran very quiet. Best suited for a 2018 Mac mini that doesn't require Power Delivery from the enclosure.