This tbh. I pre-ordered cards like the Mario/Luigi boxes at the time for 25-30 bucks each. Looking at prices of even the raw cards now, I should have bought hundreds of them, or at least two.
Then again, I also haven’t really learned despite that. I went to the Van Gogh Museum (a week too late, since they were already out of cards.. 
), so bought a copy for 80 euros when I got home. Nowadays it’s worth 1k+. I should have bought a couple of them for 80 each.
I must admit that the main reason is that 80 bucks imho already is pretty high for a single card in general. Don’t get me wrong, I buy cards for multiple hundreds USD on a pretty regular basis and my most expensive bought card thus far was 14k, but looking at it without that perspective, 80 bucks for a single card is expensive. 
Especially if it’s for a duplicate you intend to sell later on anyway.
I guess I’ll never have an investor’s, or even seller’s, mentality. 
I had a rather similar experience with the Queen of Hearts and Kabuki Art Academy cards. Both were available for sale at the same time for 3.5k and 4.5k respectively, but I only had money to buy one, so I bought the cheapest one (which I also like slightly more anyway personally). The other one was unsold for about five weeks and dropped to 4k iirc, and just when I decided to save for it to buy for 4k as well, it sold. That is the same card I paid 14k for 1.5 years ago.
The one I mostly regret, although I still don’t think I made the wrong decision at the time at all, is not buying the 20th anniversary solid golden Base Pikachu. As a sucker for all the Base Set Pikachu variants, I’d love to own it, but looking at its current price, it’s just not worth it to even save money for (I much rather save slightly more and buy one of the four missing Japanese Art Academy cards).
The retail price was 200,000 Japanese Yen, with added middleman fees, shipping fees, and import fees, it would likely have been around 2.7k USD for me at the time. Since I had just bought my first car, and my previously most expensive bought card was ; just’ 300 bucks, I simply couldn’t justify paying that much for a single card, even if it is a solid golden one.
2016 was a completely different time than now.
Nowadays I’d gladly pay 2.7k, or even double or triple that amount, for something like that (once I have that money I mean - currently a bit low ). Unfortunately, it’s worth 75-100k these days instead, last time I checked.
But as mentioned above, hindsight is always 20/20. We can’t predict the future.
Also, I was browsing through my responses for similar threads in the past, and it’s kinda funny to see something like this (which I posted in late 2018 for a thread called Have you ever significantly overpaid for an item? ):
Since I had two, I graded one of them which came back as PSA-10. Sold the other one to a friend in early 2019 or so for 1k.
So although I initially bought them for 800 USD each and it was annoying to see they dropped to 150 USD each at the time, I sold one raw copy (somewhat underpriced since it was to a friend) for 1k, and last year a PSA-10 sold for 12k.
I must admit I do kinda regret - to stay on topic - selling that copy to that friend, since he stopped collecting Pikachu cards a few years ago and I could have graded it a PSA-9 or 10 as well and sold it for thousands of USD now. But
Honestly, you can’t change it anymore anyway, so why have any regrets at all. You can’t change the past, you can only learn from it for the future.
Or to quote: “Life is the future, not the past.” - seventh Wizard Law from Terry Goodkind’s Sword of Truth book series.
Greetz,
Quuador