Primal Prime NM in box at Botcon 08... I think he was around $50 or so. Kickin myself still for not grabbing him.
For me this easy. There are 3: 1) Masterpiece skywarp at a wal-mart on clearance for $20.00. 2) ROTF leader jetfire on clearance for $25.00. 3) I owned a lot of g1's as a kid, "grew out of it," and gave the vast majority of them away. When I became re-interested in transformers years later, I was able to find around 20 of them buried in my parents' basement, but I'm sure I gave away at least 3 times that many.
Right now, the Takara deco of FE Arcee is haunting me just a bit. Everything else I missed, and wanted, I got my hands on eventually.
Same for me re: Skywarp at Wal-Mart. It was in my cart and my wife said to put it back. I could have pushed it though. I also sold off my whole G1 collection when I was in my 20s to pay for gambling debts, going out, parties, etc. Ugh. What a fricken waste.
Recall seeing those Toys R Us G1 reissues and passing on all of them. I wasn't big time back into collecting. I just took pics of them on my phone to save for later. Now I'm regretting passing on them all.
Skywarp for me as well. I never saw it for $20, and was waiting for it to get to that point. Never did, and then I never saw one again. Cybertron Primus. I remember a wall of them for $25 on clearance at a local Wal Mart ages back, and since I wasn't into the AEC series I passed on it. Now that I want one I can't find one anywhere.
Shooter almost complete for $20: At the 2001 Botcon I had spent my last $50 on a complete Ranger (Takara repaint of Joyride). Then I saw someone selling Shooter (Takara repaint of Siren) only missing his black accessories (which are identical to his US counterpart anyway) for a measly $20. If I had only had an extra $20.
I had a chance to get united bee for 25 on eBay. He had over 10 so I though I would wait bit and then he jacked up the price to 40.
Back when there were a few prototypes of Animated Thundercracker selling on eBay, I passed 'cause I figured he would still make it to retail (not an illogical assumption at the time, since Hasbro had announced him at Botcon as an upcoming release). Obviously that never panned out, and I still kick myself for not grabbing a prototype while I could. Also, these days, E-Hobby Orion Pax and Dion. I remember when they were going for clearance fairly cheaply, but I didn't care about getting them. Now I wish I had, as their price has gone up quite a bit.
2010 reissue Predaking drooled over it all the time, but never took the plunge. it's still around, but I'm not sure I want to pay $200 for it.
ROTF Mindwipe. Passed on him to get some two pack with a retool of the crap Cannon Bumblebee mold and a redeco of that crappy ROTF Soundwave just to get a chromed Ravage. Which is only an okay mold. I'm so stupid sometimes.
Had I not bought this 2-pack when it was out, I wouldn't be collecting today. As far as the one that got away, I have a list... ROTF Superion on clearance (ditto Bruticus, but I've since emptied my wallet for that mistake) MP grimlock at retail. (I know, get in line...) Henkei Smokescreen when it first came out, now impossible to find... I could go on, but I won't.
No Shit! I can't believe how much those shot up... I guess I could add the E-Hobby Anime Astrotrain to my list too. Passed on one back in the day for about 5x's less than the cheapest one you'll find now a days
Laser Rod Optimus Prime. Only ever saw one in the shops and passed on it because I assumed it being Optimus Prime there'd be plenty kicking about and I could pick one up the following week when I had more spare cash. Turned out I was wrong. But I've since got the RID Scourge version of the mold and that's good enough for me, even if it doesn't have the electronics the Laser Rod one did.