I have two that I kick myself over, and I can almost see a third playing out now.
First and probably biggest to me is Black Spiderman for $4450 NIB delivered in 2008. I had the money in hand, but just couldn't bring myself to spend that much. Figured I could always get a used one down the road, especially a red, for way cheaper. Now you can hardly find a beaten red for that. Ended up spending the money on my CV and Pinbot instead, and even together spent about $1k less than BSM would have been (CV was a partial trade), which seemed like the way better way to go at the time. While I'm glad I got CV when I did, I sure wish I would have pulled the trigger on that BSM. Now I will likely never have a Black, though I hope a Red finds it's way to my basement someday.
The other would be Neo's MM at MGC which I think was also in 2008..maybe 2009. If I remember right it was $4200 or so. It just seemed like so much money at the time for a game that wasn't brand new or in super nice shape. If I remember right I could have swung it, but it would have been tight, so I decided against it. I figured they would always be about that price, so I could find a different one down the road, maybe in better shape. Another one where hindsite is 20/20. Now I'll never own one. I like the game, but it's not near the top of my list, especially at the prices it demands now. Oh well. Of course I should have grabbed an AFM back then too when you could get them for $3-3.5k easy. Not to mention a MB. It's amazing what the prices on those top tier games have done in just the past 3-5 years.
Live and learn I guess. I have this nasty feeling that in a few years X-Men LE is going to be on this list for me. I just don't have a way to swing it right now without sacrificing half of my collection, which I just can't talk myself into. But I have a feeling if I don't do it now, I will never be able to. I see the prices doing the same as Tron LE (which thankfully I don't have any interest in), especially with one more really good update. I hope I am wrong and the prices stay down, or at least I enjoy the Pro just as much when I get to try one.