Reworked previous suggestion to include sales from more than a month ago due to the value and rarity of this item, according to Blaze.

https://backpack.tf/classifieds?item=Fireproof%20Secret%20Diary&quality=6&tradable=1&craftable=1&australium=-1&killstreak_tier=0

Buy Orders start at 50 keys.

No Sell Orders currently exist, so I'll be using my judgment to determine a "limit".

No Bot.tf Sales in the last 30 days.

Marketplace.tf Sales:

https://ibb.co/PD2hZh9

1 Sold for $114.99/ 1.84 = 62.49, ~62 keys.

1 Sold for $120/ 1.85 = 64.86, ~65 keys.

1 Sold for $123/ 1.89 = 65.09, ~ 65 keys.

While it's true that the Halloween season is more than likely the cause behind the gradual inflation of the item's value, it's worth noting that even the SCM reflects these sales, being around the same general price range, as shown here:

https://steamcommunity.com/market/listings/440/Fireproof%20Secret%20Diary

https://backpack.tf/profiles/76561198063101663#!/compare/1589328000/1602201600 - sales at 40, 40, 48, 48, 53, 53, 56, 67, 85, 98

https://marketplace.tf/items/tf2/5605;6

https://backpack.tf/item/957813686

https://backpack.tf/item/950980000

https://backpack.tf/item/949173838

https://backpack.tf/item/900786002

https://backpack.tf/item/895793041

- other bit older sales which should be considered $75 (41 keys), $86.13 (47 keys), $89.99 (48 keys), $80 (44 keys), $102 (55 keys), $95 (51 keys), $95 (50 keys), $89 (47 keys)

https://backpack.tf/profiles/76561198043625371#!/compare/1601510400/1601856000

https://backpack.tf/profiles/76561198144097470#!/compare/1600646400/1603497600 - sold for 60 keys

https://backpack.tf/profiles/76561198062611321#!/compare/1601078400/1601164800 - bulk

https://backpack.tf/profiles/76561198192740165#!/compare/1602979200/1603238400 - sold for 52 keys

https://backpack.tf/profiles/76561197998319899#!/compare/1598486400/1600992000 - bulk

Most likely a temporary value change, but it's been 6 months since it's had a price change, so why not? Going for 50 - 65 keys.