The newly introduced Unique Item Collection Tab in Path of Exile allows you to store one of each unique item in a single tab. This allows you to potentially storeover 900 different items in this single tab, but most players are unlikely to get anywhere close to that number.
The Unique Collection Tab cost 140 points in the shop, which means that it cost about $14 to buy. If you want to keep a lot of different unique items, then it might be a good choice, but you will still need other stash tabs to keep multiple versions of the same unique item.
If you are abou collecting uniques, then it's a great deal, because most people will find 5-10 stash tabs will unique collections per league. However, if you typically just vendor all of them anyway, then this stash tab is unlikely to be worth it for you.