If you don't plan on connecting and disconnecting the extender more than a couple of times, you could get away with ENIG plating on the edge connector instead of hard gold.
JLC makes edge connectors with ENIG plating for quite cheap. I can't see the size of the board, but a 200 x 200 mm board with 2U'' ENIG, edge connectors with edge chamfer is 100$ shipped for 5 pcs. It won't get cheaper than this.