Rarity:

23 in the wild

4 on OP

http://backpack.tf/unusuals/139/Modest_Pile_of_Hat

TF2PC consensus: ~6

http://www.tf2pricecheck.net/Thread-PC-Vivid-Plasma-Modest-Pile-of-hat?page=2

Completed Trades:

http://www.tf2outpost.com/trade/7975593

SOLD to me for 5.5 pure :D

http://www.tf2outpost.com/trade/8142214

SOLD for 6 pure (after he was a *$% to me for offering 5.5 lol, w/e i got mine :P)

before: http://backpack.tf/id/76561197984548549?time=1359014400

after: http://backpack.tf/id/76561197984548549?time=1359100800 (+6 buds)

Open Trades:

http://www.tf2outpost.com/trade/7426317

Notable offers:

-5 pure

-5.5 pure

-Planets Killer Exclusive + Vintage Tyro Orb. Fire (~6.7)

-Stormy Bonk Boy + P. Energy Dixie + 20 keys (~6.7)

-"If you'd be interested in a mix of pure and PayPal, I'd offer around 8"

-nuts tc + nuts cannonball (~5.7)

-misty skull el jefe (~10) i guess this one is somewhat tentative

http://www.tf2outpost.com/trade/8059973

Notables:

-5 pure

-burning ze goggles (~5.6)

-Purple Energy Noble + Planets Little Buddy (~5.5)

-tentative nuts tc + nuts cannonball (again)

-I was going to offer, but he seemed firm at 7 so i didn't bother :P

http://www.tf2outpost.com/trade/7852864

-5 pure (2x, once by me, but then he was rude so i didn't up my offer. :P)

-5 pure + 5 keys (retracted)

-secret handle + BMOC + bud + SF sandvich (~5.7-6?)

-searing plasma voodoo juju (?) I think it's 1:1 :L

-Eerie honcho + 1.25 buds (~5.75)

-burning ze goggles (again)

So as you can see, these get 6+ unusual offers and sell for about 6 in pure. I feel I got lucky to buy mine for 5.5 :D