Its hard to say what my best find is, as I've had several good finds, last year I bought probably $1000 worth of stuff for $175 on craigslist. but my best deal would probably be a Garage sale where I got RID Prime, RID magnus, RID Scourge, Armada Nemesis Prime, and 2 each of Ehobby T-cracker and Skywarp and 1 reissue Starscream for $25 bucks total all MISB this was almost 4 years ago though.
About 10 years ago amazon had a clearance, and I bought a bunch of optimal optimus' for $8.25/ea. Etoys going out of business clearanced off primal primes for $17/ea. More recently, Toysrus clearancing off 20th dvd prime for around $20/ea.
Classics UM vs Skywarp 2-pack x2 for $13 each on clearance Classics Grimlock, Rodimus, and Cliffjumper for $5.88 each on clearance Classics Devastator WM Exclusive 5-pack x2 for $25 each on clearance. Universe Classics Prowl @ KB store closing for $6.50. I wish I would got more of the Devastator and UM vs Skywarp packs because when I bought them, Wal-mart and Target had tons of each.
I got G1 Jetfire for free back in 1991. A high school classmate came to my house wanting to trade his old Transformers for some of my football cards and I offered a Barry Sanders rookie for it and he said he would think about it but told me I could hold onto Jetfire for a few days until he made his decision. Since we rode the same bus he figured if he did not like the deal I could always give Jetfire back to him. What happened ? He didn't show up at the bus stop the next day, or the next day after that for that matter. Didn't see him at school either. Never saw him again for that matter. Weird as hell, but true. I still have that Jetfire too-beat up a bit but still damn nice.
About three years ago, I got a 95% complete, dead mint U.S. Computron gift set for $50 BIN on eBay. I spent a couple of dollars to get some of the guns that were missing. I'll never beat that deal.
Today, actually, was when I got a junker Megatron who's only really missing the mini-con, missiles, and horns, and is otherwise near-mint. and I got him for free. I'd found him at some guy's house years ago, never thought I'd ever own that very same figure someday (and although I always did want and Armada Megs, the figure I'm talking about remains the only one I've ever really held in my hands) I'm gonna hit the 'bay later to pick up his stuff if I can. You can't have Armada Megatron look like himself without his horns!
i live in england, but rumour has reached my ears about 'bargains' and 'deals' in far off lands. It'll never catch on here.
Cybertron Primus for $25 off amazon plus $5-$6 shipping. Also found a Classics Jetfire and a Classics Optimus Prime for a combined cost (with shipping) of $40 in mid 2008 (considering how rare it was/is to find them for a good deal). Both of them were MISB. Best deals for me.