The short version: 102 Genesis/NES games for $150 ($1.47 each).
The long version:
1. I never owned a Sega Genesis until I found one at Goodwill recently. No cords or controllers for ten bucks. I had bought a random Genesis power adapter and controller before just in case I ever came across one. Now all I needed was an av cord, which I bought from racketboy. Oh, and some games naturally.
Someone was offering 31 Genesis games for $60 bucks on my local craigslist. There was just a blurry picture in the listing so after getting the seller's number, I had him tell me the titles. I was expecting about 20 Maddens but surprisingly he named a bunch that sounded interesting enough to begin a collection with. I offered him $50 and picked them up the next day.
Turns out there's not a lot of "hidden gems" or "cheap favorites" but I'm sure there are plenty of decent titles we'll have fun with.
One awesome surprise- two copies of Splatterhouse 3, recently profiled in a rarest and most valuable post. One is CIB in very good condition and the other has a big, fat Blockbuster sticker on it.
Here's a picture of all the Genesis games (and you can find a bigger one here.)

2. While I was at the seller's house picking up th games, he showed me some other video game stuff he had. He's got a passing interest in retro games as a hobby but mainly just comes across things at storage facility auctions and turns them over. He showed me a container with about 50 NES games in it and my eyes about jumped out of my head. He was going to list it on craiglist for $100 or $125.
The whole way home I was thinking about those games. Sure I was excited about my new Genesis titles but the NES is probably my favorite system. It's what I grew up with during my teenage years. That guy had a whole bunch of games I didn't have that I really wanted to play. I got home, gave my wife a chance to talk me back into my senses (she was actually very encouraging and all for it), and called that guy back about them. He said he'd give them to me for a $100 instead of listing them and I picked them up the next day.
Turns out there were 68 games. Nothing rare or particularly valuable, but tons of "hidden gems" and "cheap favorites." Sure, there's a lot of junk too. Every other game reminds me of an Angry Video Game Nerd ree-view. But there's a lot of awesome stuff too. What's really cool is that there are only seven doubles from my previous collection. All-in-all I'm more than happy with the purchase.
Oh, and the guy turned up three more CIB Genesis games and gave me those on that second day too.
The new additions to my NES collection (and a bigger picture here).

Sorry for the long post but I figured if there's any group of people that's going to get as excited about this sort of thing, they're hanging out here.
edit - bad math