The single most expensive item I own is a gold coin from Imperial Rome, an Aureus of emperor Antoninus Pius. I bought it about three years ago when I was just starting to collect ancient coins. I came across this particular coin on a "regular" gold & silver bullion site in my neck of the woods, for 3K. Not knowing too much about it, I bit the bullet (which is actually an incredibly stupid thing to do, akin to gambling). Turns out it's very real, mint state, and worth about 2K over what I paid for it right now. I have since continued collecting ancients, especially Romans, and by now know the entire history of Rome and all its emperors in detail. Which again underscores how incredibly stupid it was to buy something so expensive without decent prior research. I was just incredibly lucky that an actual reputed bullion dealer apparantly had come across this coin and got rid of it far under what it was worth.

I don't expect to be able to repeat this feat, but I'm definitely on the lookout...