I've been using two 512 MB 24x Lexar CompactFlash memory cards with my D100. I'm upgrading to a 1 GB card, and I'll keep one of the 512 MB cards for backup.
I'm going to put the other 512 MB card on eBay with a "Buy it Now" price of $85 (they currently sell for about $150-$160 at discounters like B&H). The card has been my backup card and has only had about 500 write cycles. Note that this is not a WA (write acceleration) card.
The card has been stored in the original plastic case and is in perfect condition functionally and cosmetically. I'll take $75 for it if anyone wants a good deal before I put it on eBay in a few days. Shipping will be free.
