Personally, my fiancee bought me a G1 Shockwave with working electronics for a dollar at a junk fair

 

Got a g1 gears for 4$

 

I got Cybertron Metroplex, missing only his minicon, for $12 over the Summer in a flea market in Tennessee.

 

I love metroplex, I managed to get a G1 Loose Complete for $65. The Cybertron one looks amazing as well

 

AoE Hound at a Japanese hobby store for slightly less than a retail Deluxe. He comes with all his weapons and instructions. Truly a fantastic deal, especially since he himself is incredibly fun and well engineered.

 

Movie 1 Arcee for a couple of dollars

Im changing my answer ether the 80 usd MIB Knights of Unicron set or the 92 usd MISB Powered Convoy set, or the Prototype BH Ripclaw for 14 USD

 

A few came to mind:

-WFC Bumblebee for $4.96
-Combiner Wars G2 Menasor for less than $40
-Takara Legends Devastator (the one from the Lost Age line) for about $20

 

10 US for WST Hot Rod MSIB, MP Blackarachnia for 86 MSIB the week she as out, 70 for LG EX Ginrai (additional 60 for the inner robot + head), 50 for LG EX Big Powered, 20 for Movie Advanced Evasion Optimus, and I’m getting Telemocha BA sometime around this week on the FB marketplace if the seller manages to find it.

 

A mint RID/Car Robots Megatron for a under a hundred dollars about a couple years back. I was seeing loose ones for over a hundred, and other boxed ones closer to 200.

The thing was so old that the plastic bands basically disintegrated when I touched them, but the figure was perfect. I was mostly out of Transformers collecting at that time, so I was in amazement at what I had been missing out on from that era. They just don't make 'em like that anymore.

 

Takara t30 Windblade, Slipstream, Nightbird Shadow, t30 Chromia, Funpub Strika, a windcharger (not prime wars) a chromed 3rd party Seaspray and another figure that slips my mind for $40. I don't know if I can ever beat that.

 

Nice. I got LG Slip and Nightbird for 25 each at December last year.

 

Classic Prime for $10
Siege Megatron for $20
Siege Shockwave for $25
TLK Hotrod for $7
TR Octone for $16

 

I bought a bulk lot on Ebay for $50. It had:
Both versions of RID 2001 Sideburn, Prowl, and X-Brawn (mostly complete)
Rid 2001 Optimus prime and Ultra Magnus (Mostly complete)
Supreme BM Cheetor
Complete BW Optimal Optimus
A few other figures that escape my memory.
Enough Digimon digivolving figures to sell for $300 on Ebay individually.

 

I got a free Titan Metroplex from amazon.com - they sent a duplicate one in error and told me not to bother returning it and keep it as a free gift

 

Siege Sixgun for $12.91 on Amazon.

 

I ve got several bit I'll start with me getting an "incomplete" g1 metroplex with six gun for like 8 bucks on ebay. When it arrived I found his fists in his chests with the other white pole and the tower thing, scamper with his gun in his hand in one leg, slammer in the other. So I got a complete metroplex for 8 bucks. I felt like playing the lottery.

 

Found g1 broadside at goodwill for $1.
Found transmetal prowl and tm2 dinobot for a combined $3 at a flea market last year.
Earlier this year at a different flea market I came across a box of toys that were broken or missing pieces. Dug through it and found all the pieces to terragator and got him for 50 cents. The best part was none of his joints were loose.

 

I got Striker Optimus Prime (the Asia exclusive version) for £70 just a couple of weeks ago, a bargain considering he usually goes for ~£250. A genuine copy and in excellent condition too.

I remember getting similarly excellent deals for MPM Starscream and the Buster 2-pack version of Jetfire.

 

I used to go to garage sales, and I found a garage sale with several loose complete g1 figures including a couple seekers, megatron, motormaster, a very nice condition swoop, and a few others along with a small-sized moving box 2/3 full of 1980's gi joe figures and weapons all for around $30 if I remember correctly. I posted about it on this site when I found it.