There are many unique items in the Path of Exile, unique items are meant to be less powerful overall than high-level rare items, but offer unique gameplay and allow special character builds to be based around them.
Unique Items have a predefined set of attributes on them, and since most of them are global drops, even the best Uniques won't cost a fortune. Obviously, items with perfect rolls can cost several times more than the same base with rolls on the lower end, but still what makes some Unique Items extremely expensive is primarily the difficulty of obtaining them.
There are several types of Unique Items which can be found in Path of Exile:
Regular Unique Items - which can drop from any source, as long as you are in a high-level zone. Most expensive Items in this group will cost between one or two Exalted Orbs, for example for decently rolled Shavronne's Wrappings or Hyrri's Ire;
Unique Items with drop restrictions - a group that includes League-restricted Items, Fated Items, Boss-specific drops, specific Divination Card rewards. A number of Items that can't be found on regular Maps are heavily demanded by high-level players. In this group, among others, you can include drops from Bosses like Uber Atziri, Shaper, Elder/Uber Elder, Aul, Cortex, Breachlords. The most expensive Items that can be found in the Path of Exile but will require you to farm a very specific content are Farrul's Fur and Headhunter.
Legacy Unique Items - Legacy variants of currently obtainable Uniques, which have different stats (or their values). These Items no longer drop in the game, nor can they be obtained in any way. It was briefly possible during the Legacy League from Ancient Reliquaries. Obviously, Items with higher base stats are the ones that will be significantly more expensive. A few Legacy Unique Items can reach the price of several hundred Exalted Orbs, with solid double Corruptions getting close the price of original copies of Mirror-service Rares.
When we enter the Path of Exile, these unique items can help us gain an advantage in the game, and we can also use Poe currency to strengthen these items. If you don't have enough Poe currency, you can check how to get it from the following article:
What Are The Tips For Obtaining Poe Currency In Path Of Exile?
