My best deal was also the one that cost me the most!
I got into military surplus awhile back and most of my collection centered around a half dozen Mosins of various styles, types, etc. An awesome deal at Cabela's finally got me a mint '53 SKS for less than I expected, but I still didn't feel like a true collector.
Then I found an obscure ad online here in Omaha. I checked it out and to make a long story short...I came home with a 1897 Arisaka Type 30, mum intact, not duffle cut with original bayonet and sling. The gun stock had some "in field" carvings and was in excellent shape! To make it better, I eventually got the first hand story from the Soldier of how the rifle came into his hands. I paid $600.
Another great deal was a Mossberg M44US(b) I found on the used gun rack at Cabela's for $150
My lowest priced deal was trading an old RC truck (was looking to get maybe $150 out of it) for a mint Browning A5 12 ga.
There's some awesome deals to be found if you're persistent and patient