The Mann Co prices that stuff at like $5-$15 for hats, depending on the hat. Yet TF2 players price them at $2 or less while SUPOR KOOL UNUSUALZ go for $100+. What the forget.
Reasons:
1) The mann co. store supports the game, no wonder they over price things.
2)You can get them from drops, that takes the value down a notch, even though hats are rare. Value goes down even more if it it's more common.
3) It's because they're rare. Rarity is what determines prices. What ever you're selling isn't rare.