The fee is going to be 1% of your asking price. So if you want to ask a few million for your Gurgox Hammer... you'll pay a big listing fee
On the other hand, if you're feeling charitable and you decide to list this rare item for only a few thousand... you'll get by with a nominal fee.

Rarity and such has no bearing on the asking price. We know of some green and yellow items that are (shamefully) a lot more rare than the most sought-after pinks. These may be worth a lot more to a collector even though they are not the best stats, etc.