Wow, that was a nice find!
Tropic of Cancer is actually one of my favorite novels, and I got that fifth printing on eBay for around just ten bucks and shipping -- it was horribly described and had a stock picture. The seller sort of got across its age, and I took a gamble on it. It's kind of hard to say what it's worth, since book collecting is way more niche than, say, video game collecting. I'd love to get something close to the price I have listed though.
I used to be way into collecting books, and you do come across a lot of early printings floating around the thrift circuit. For instance, I got this one for a dime: eBay Auction -- Item Number: 320795091654. $300 is a very valuable book, though -- nice work recognizing what you had!