Firstly, green confetti is a first generation hat effect, meaning, it's is supposed to be very desirable. However, many hats that are listed to be worth more than the effect are 2nd or 3rd gen effects. Another reason that this hat's price needs to be updated is that the price hasn't been updated in over a year, thus increasing the need for making the price change even more.

There are very few trades involving this hat, none involving the green confetti effect.

http://www.tf2outpost.com/search/263998760,2

http://www.tf2outpost.com/search/263998760,1

Also, even though there are plenty on the steam market, none of the listed items have this effect, and higher tier effcts are selling for upwards of fifty dollars.

http://steamcommunity.com/market/listings/440/Unusual%20L'Inspecteur