I have some spare keys that I want to trade for an unusual hat.
What are the prices of all the unusual hats? And is there an unusual within the price range of 5-10 keys?
There are entire dedicated websites for this and thousands of unusuals, nobody is making a full list for this here, plus the prices are constantly changing so it would not be acceptable for this site.
TF2 hat pricing information is available at a number of websites, including tf2spreadsheet.blogspot.com/, backpack.tf/pricelist, and tf2unusual.com.
For unusuals in particular check out backpack.tf/unusuals. Based on that site, even the cheapest unusuals run 10-15 keys, and most are expensive enough to be measured in earbuds.