I don't really know exactly but I lost about £20 when I bought my PS4 as I had just been reimbursed for a month's train travel (Thanks, Thameslink). Scored a PS4, Horizon: Zero Dawn, an extra controller and 2 months of PS+

probably a heavily discounted baked good at a discounter dfjlfdgjlfjhf

Bad Rats on Steam for... I guess about 50 cents?

a 20 cent piece of candy is up there, I think

A pen for $1.00. Buy 1 free 2

Outside of a boring answer like penny sweets. Probably the cheapest surprising answer would be a Lacoste pair of leather shoes I purchased for £3. I've got so much use out of them.

I've made a few $0.00 purchases in my time. Coincidentally I just made one last night. =p

that I can remember: a Kit Kat for 89 cents

idk but i bought bloons TD6 for 81 cents on steam a few days ago which was a really good deal

I bought Last of Us Remastered for $10 which came out to free when I used my Amazon points.

Anything that I ordered in Aliexpress before the pandemic. Since I wasn't receiving the stuff after 3 months I asked for a complete refund which they accepted. But some weeks later, I received it all.. So I got it for free 😅