I will do a quick recap of some of the stuff I have said already but I will also go over other stuff.
For the most part the newer the weapon conditions are the better and higher they will be priced.
The higher the rarity the more money it SHOULD be worth. I say it should be because there are always exceptions like supply.
Just like in real life, supply has a lot to do with this games economy. When a brand new case comes out the supply is extremely low for the skins and the cases. Lets use the newest case as an example. Operation Wildfire was the newest case that has came out, when the case first came out people sold the case for over $40 because it had new skins and the demand was WAY greater than the supply. Certain weapons are also worth more when they're Battle-Scarred but keep in mind that is very rare. The reason why they are like that is because people usually use the Battle-Scarred weapons for trade ups (I'll get into trade ups in a bit). Since thy are being usd to make trade ups that means there are less of them and that itself isn't enough to make it cost more but, Valve tends to take cases off from being able to be dropped and since a lot of the Battle-Scarred skins were used for trade ups they become more rare than the Factory New skins. Not all skins have their price go up because the skin may be made available in another case or as a drop after a game.
Trade Ups are used to get a weapon of a higher quality, it takes 10 weapons of the same quality to get a random skin of the next quality. Trade Ups can be used to get a weapon of a low float. if you mainly put low float skins for trade up you will most likely get a low float weapon and same with higher floats as well. Putting 10 StatTrak weapon skins will give you a high (almost guaranteed). Some weapons can be put in a trade up to certain weapons or have a chance at least. Those weapons that trade can trade up to certain weapons that are desirable will have an increase in its price like the m4a1-s Knight. Recently that weapon has gone up in price because its crucial for the Dragon Lore trade up and the low float Knights have been selling for over $230 USD.