My information might be a little outdated but, just opening a knife is pretty rare to come by. A key to unbox a crate costs $2.49 and the box could cost anywhere from $0.10 to $30 or you can get them as a drop by playing on a server (drop isn't guaranteed). There are many different type of knife skins and wears. Undesirable skins such as safari mesh or urban masked will cost significantly less than a pattern like a fade. Different wears will also dramatically increase or decrease the value of the skin. In order : battle-scarred (rotten), well-worn (slightly-rotten), field-tested (almost at its expiration date), minimal-wear (pretty fresh), and factory new (really fresh). However, there are slight differences in the condition of each wear. Sometimes a glitch can occur which might make a minimal wear (few scratches here and there) look factory new, and its value might be closer, if not the same to a factory new version. Patterns can also change the price. For example, the crimson web skin has crack/web patterns. If the crimson web knife has a big or a few visible webs on the front, it is considered much more valuable. In my example with the fade, somewhere around 50-70% fade is not desirable hence why you will find many of them on steam community market (max you can set them for is $400... steam takes a percentage from the sale). A full fade (90-100) is usually found on cs go trade sites where you can set the price to your liking. There are different types of fade like pink or purple, but I won't bore you with that.

As I mentioned earlier, a key costs $2.49 (if you purchase it directly from steam), but you could get them cheaper buying them from a trusted seller for as low as $2.00 or even lower if purchased in bulk (100+). In my case I sold all my keys in bulk via paypal gift and netted $1500. Pretty low, but I sold 1100 keys to just one buyer so I gave him a pretty big discount.

Just think about the cs go market like business in the real world. Rare car in mint condition? Millionaires want it? Its going to be expensive as shit. Rare clothing piece? Its going to be expensive as shit. Hyped up sneaker like the yeezy? Its going to be expensive as shit. Extremely limited? Its going to be expensive as shit.