What makes CryptoPunks special
There are a few things that make CryptoPunks special as a project.
First of all, it's one of the earliest NFT projects on the Ethereum blockchain. It started months before the launch of CryptoKitties, a popular cat-breeding blockchain game/NFT project. Being one of the first projects of its kind gives it value.
Second, only 10,000 CryptoPunks will ever exist. This creates scarcity among collectors.
Some CryptoPunks portraits may be more valuable than others based on their attributes (or lack thereof). There are just nine aliens and 24 apes, 44 wear a beanie, 48 wear a choker, 78 have buck teeth, and 128 have rosy cheeks. Eight CryptoPunks have no defining attributes, and one (#8348) has seven attributes.
A CryptoPunk with multiple rare attributes can be worth a lot. For example, CryptoPunk #7804, an alien (9) wearing small shades (378), a hat (254), and smoking a pipe (317), fetched more than $7.5 million.
But rarity isn't the only factor in how much a CryptoPunk is worth. Some portraits are desired merely because a collector likes the look.