Yeah, it's hard to place a price tag on it. You're not going to see large groups of people wanting this card, mainly because the models of Mac it's compatible with aren't particularly collectable. That said, if you time the sale right, you might find one guy that just loves his Quadra 630 (or whatever) and would love to have this, and thus you might get a little more because of it. All you can do is drop it on eBay and see what happens.
Not a really popular item -- these are for the Performa series, which are not really popular collector's machines and don't normally sell well on eBay. An accessory like AppleDesign speakers or a Iic flat panel display are very popular items and normally sell very well on eBay. There aren't many "accessories" that are hot sellers on eBay. Mostly people are looking for the machines, like Classic Macs (128k, 512k, Plus, SE, SE/30, Classic, Color Classic), and IIfx, Quadra, etc. Those are all very popular.
There was a "prototype" Clear Lisa Widget hard drive that sold for several thousand dollars on eBay last week, but it was a prototype.
For this kit, if it is in good shape, I would say $15-20 max, but again good luck finding somebody who genuinely has a use for it, most people have chucked their Performas by now.