Through reading this topic I noticed that I have the biggest losses of you! I am a Libra (well, happy birthday to me tomorrow!) so I get excited easily and often do things without deeper thinking or on a whim.
Here are the bigger ones I can think of currently.
Recently I've paid 50 $ for Maximo because I was dying to play it and I thought it's the only piece in Czech based on the fact the site was advertising it as an uber rare game and through my finds I have not seen it anywhere else. Then I was so surprised when I found another shop selling it for like 13 $ .
Luckily for PS2 I've found the right sources finally so hopefully a loss like this won't happen again.
For PC it's at the moment every game which I did not manage to play but the worst is probably Battleforge. I pre-ordered this one for 50 euros in hope to play it's beta but in fact I have never played the game and soon after release it became completely free to play.
Another but even bigger waste of money for me was buying the Vita.
Well I have to be more careful with my future purchases!