Maybe $50 to $75. 8-bit cards that only do 640x480 aren't highly sought after by collectors. Those are the cards you buy because you have a II, IIx, IIcx or IIfx and just want something with video output. So, you spend as little as possible.
Given that a new old stock Macintosh II Video Card is listed for $75 and hasn't sold and used ones sell occasionally for $50. That tells me that collectors won't attribute much value to it being new in box. As the SuperMac card isn't much different than the Macintosh II Video Card or Macintosh II High Resolution Video Card.
It might hit $75 since it is like those cards after they get their VRAM upgrade.