Proof:

I purchased this hat for a Massed flies Pyromancers Hood which is priced at 66 keys, although i was asking for 70 at the time. I also added sweets which added up to be about 3 - 4 keys. http://imgur.com/a/gdfPW

I had very good amount of offers in the few days i owned this beautiful hat, some ridiculous but the most recent was this:

This set was a Burning Magistrate's Mullet and a Cloud 9 Le Phantom. His claim that it was worth 130 keys was ridiculous, BUT the offer was still 80 keys (40 for each) http://imgur.com/3d5EgKp

The next offer which i almost took was this clean Aces Killer Exclusive which has an old price of 100 keys... although they are asking for a bit lower on outpost, most asking for 70 keys, this was a very substantial offer and he had even added items as sweets worth a few keys. http://imgur.com/OYcDVqz

The last offer i received was a Poisoned Shadows Harmburg, although this hat was unpriced, it was recently sold for about 95 keys in unusuals. He asked me to add 10+ keys in items and I said I would be able to, but has yet to get back to me http://imgur.com/tUBXNES

This Harmburg is in the hands of a new owner asking for 100 keys (http://www.tf2outpost.com/trade/26005444), and although it was my best offer, i did not take it because of it being a spy hat, that is the only reason.

I eventually sold it for a Miami Nights Crones Dome because i already have a second crones dome and would like to start collecting them.

The Miami Nights Crone's is priced at 88.5 keys and although i added sweets the deal was still fair

http://imgur.com/a/xys0v

TL;DR: So heres the math:

Offer1- Massed Flies Pyromancer's Hood ~ 66 keys + 4 keys = 70 keys

Offer2- Burning flames Magistrate's Mullet ~ 38 keys + Cloud 9 Le Party Phantom ~ 40 keys = 78 keys

Offer 3- Aces High Killer Exclusive ~ Multiple asking 70 keys on Outpost, but priced at 100 = 85 key middle point

Offer 4- Poisoned Shadows Harmburg ~ Currently asking 100 keys on outpost, Previous owner asked 100, Previous owner to THAT sold for 95 keys ~ Asked me to add 10+keys = 87?

Final Sale = Clean Miami Nights Crones Dome (priced at 88.5 keys) with sweets added

Sweets = 4 keys (going off SCM prices for some items)

Final sale was approx 84 - 86 keys

70 + 78 + 85 + 87 + 85 = 405

405 / 5 = 81 keys

Price fluctuated from roughly 78 - 84 keys