I have one in my wallstreet as well.
ZIP was pretty good for backing up systems at the time, since it had more space than floppies(1 Zip 100mb=about 100 floppies).

I think that maybe you would get about $25-35 for it.

Oh, and iomega still sells zip disks.
Here

(Sorry for bumping an old thread)
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My Collection:
Macintosh Color Classic
Macintosh LC III
Power Macintosh 6100/60
Macintosh Performa 6360
PowerBook G3 WallStreet PDQ