There are a variety of different items that can be considered amongst the rarest cs go skins available. Obviously personal preference is an important factor when choosing, but for those looking for something rare, there are many options available. A little bit of internet research will show an individual a wide range of styles and price points. One of the rarest and highest in price is the Stat Trak (Trademarked) M9 Bayonet Crimson Web. Some sites show this being hard to come by and also expensive. One site posts the price for it as around $1800 or even higher. Equally rare is the Stat Trak (Trademarked) Karambit Fade. This comes in a bit lower on the price point scale though, and some sites list it at a price of nearly $1200.
PC Gamer Pro says that items are subject to a $400 max price, but many are sold for much higher amounts depending on how rare they are. For example, an Ak-47 Fire Serpent was posted about on their site for $4500, even higher than the previously mentioned Stat Trak M9 Bayonet. If you are looking for a rare CS Go Skin, be sure to make sure that the seller or dealer is reputable since there are certainly some sites and sellers on the internet that are not trustworthy. Also according to PC Gamer Pro, if you are looking for something rare but cannot afford such a high price, perhaps look into something like the FAMAS: Spitfire. They list a factory new version as going for $225, much less than any of the previously mentioned items. Although it is still expensive, it is definitely more in the appropriate price range for many individuals.
If you are looking for something rare but even the Spitfire is too high, there are absolutely additional options available at a lower price point. A good example of that is the Five-Seven: Case Hardened. This was released in November of 2013 and according to some sites it will retail for only $73, much less than a majority of the rare items. Another good item to look into for a low price would be the R8 Revolver: Fade. When searching online, this item seems to be available for only $43. Overall, when looking for a skin, be sure to do research to find something that is appropriate in price compared to the rarity. Also, be sure to double check and look into the seller so that you do not deal with a business or individual you cannot trust.