A price is hard to think of for this part. On one hand, it is super-rare, and it makes the B210 hatchback look super-awesome, on the other hand, it would take some fabrication work to get it to work on a B210 hatchback. (though I think that finding some round taillights that are close enough to work out wouldn't be impossible.)
Ebay is probably the best way to go - My guess is that it would sell for at least 75, and maybe a couple hundred if some B210 guys with deep pockets get into a bidding war. As you can see from this thread, this is a part that gets B210 fans all hot and bothered. Someone on Ratsun might even want it to make a fiberglass mold ...though I might be overestimating the demand for this part, since I personally think that this is the coolest photo of a B210 ever: