I spent over $100 for my Sniper unusual in TF2. But it's sort of my dream hat that I will never trade away, so I thought it was worth it. Before that I had a cancer unusual that cost $15.
Also spent like $60 on a Strange Festive Sniper Rifle. I already had a Strange Sniper Rifle that was Hale's Own, it's not like I paid for anything but a slightly looking, much more expensive skin. Then I did the same thing again when Gun Mettle happened, bought another $10 Strange Sniper Rifle skin.
I would love to get a Strange Villain's Veil for Sniper, but those cost around $10. I'd love to get an Australium SMG, but I can't verify spending ~$30 on a secondary I hardly use, when I already have two other Strange SMGs (because I bought a Strange Gun Mettle SMG), all for making jokes about CoD when I pull it out.
Plus there's things that I subconsciously want but decided to put an end to because of cost. An Australium Sniper Rifle is nice, but those cost ~$80, and I prefer how my Festive looks. A good Unusual Villain's Veil is over $100, closer to $200 honestly, and of course I'd go all-out and make it strange for another $10. I still want a Professional Killstreak Kit for my Sniper, but those kits are usually $40+ and I'm not a huge fan of killstreak items.
My dream Sniper loadout (my Green Plasma Snaggletooth, a flashy Unusual Strange Villain's Veil, Prof. Killstreak Australium Sniper with Strange Parts Headshots, Prof. Killstreak Australium SMG because why not, and my melee which actually does not exceed $1) is around $400. I've spent $200 dollars on it already. That's not counting that half the stuff I've bought before don't fit into my dream loadout.
That's actually my biggest investment in a game, and I don't terribly regret it since I love TF2 that much.