The most valuable comics I ever owned were X-Men (when they were "All-New, All-Different," but before they went and turned all uncanny on us) numbers 94, 95, 96, 97, 98 and 100, plus the first and second issues of Conan the Barbarian. I once bought Cerebus #5 for a quarter at a flea market, then traded it for the first issue of DC's The New Teen Titans-- which, if I'm remembering correctly, was much more popular than X-Men at the time. The cover of which I managed to dot with a sticky piece of a Chewy Sweettart. So if you have a The New Teen Titans #1 with a tiny pink speck on the cover, it's from candy. And it's mine! And I want it back!
How much did I pay for that broken run of early X-Men? I believe it was around 15 bucks, plus the use of my left-handed fielder's glove during PE. This was before we had any idea you might sell a comic book and become fabulously wealthy and live in a gilded palace. Not long after my friend and I completed this transaction, I saw a Mile High Comics ad-- and X-Men #94 was going for $60! That seemed insane. How was it even possible? It wasn't even the first issue! Let's just say my friend was a little... unhappy... when I told him the news. You'd think he'd have shared my joy; after all, if it hadn't been for him, I'd have never known what a Wolverine or Nightcrawler was.
Learning they were approximately as valuable as uranium didn't stop me from taking those X-Men comics to school and passing them around during study hall. Or reading them dozens of times. Eventually a couple of comic book shops opened in our town and I learned you were supposed to seal your comics in vinyl sleeves and store them in acid-free "long boxes." By then I was addicted to comic books and justified my illness to my parents by telling them comics were "valuable collector's items." I learned how wrong I was when I got sick of them and tried to sell them at a comic book show. Apparently, my "key issues" weren't in very good condition. I could trade them for more comics-- yuck-- or sell the entire longbox for about $100. There went a complete John Byrne run on Fantastic Four, almost all my X-Men (except, strangely enough, a few issues from the "Dark Phoenix" era, which I still have hidden away) and close to 100 issues of Amazing Spider-Man including #252.
What do I have left of my first comic book collection? Well, a few odd issues of Cerebus, some New Mutants, Kitchen Sink's The Spirit reprints... and that's about it. During the comic boom of the early 90s I added a lot of Valiant titles and overpaid for New Mutants #87, only to watch it steadily decline as Cable-fever burned itself out through massive print runs and the realization that these stupid things aren't even scarce! I even have X-Force #1, and all the premiere issues of those Image titles you used to hear so much about.
One day, a million idiots will simultaneously attempt to sell their pristine copies of New Mutants #87 and the resulting letdown will cause a miniature existential black hole which will suck the last remaining hope of comic book riches right out of their souls.