The monetary value of the skins, like anything in life, is what a buyer is willing to pay for it. The skins that cost more are rare or they look good or both. You will get random skin drops just by playing the game. However, the 'better' skins are only found in crates that you open with keys.
I'm not sure how much you know about Valve's crate system but it's basically gambling. You get crates to drop for you while playing the game. You have to buy a key from Valve (or other players) for up to $2.49 each. One key opens one crate. Each crate has a pool of skins and opening one will give you one of those skins. If you're lucky, you can get a very rare one that can cost hundreds of dollars. Most of the time you will get something that is worth far less than the $2.49 key you spent to open the crate in the first place. Like I said, it's basically gambling. Knife skins are the most expensive because they have the lowest chance of 'dropping' from a crate.
The good news is, all this is purely cosmetic and does not affect the game in any way. You are free to ignore it entirely and just play normally.