Jason,
there are whole forums on this as you can imagine. There are formulae for figuring out what a domain name is worth, but it's still like anything else: it's worth what someone is willing to pay for it. For established sites a good rule of thumb is a site is worth 10-20 times monthly revenue that is generates, but this depends on whether it's content driven or just a typo that generates hits.

I suggest if you aren't going to develop the domain name and want the revenue, rather than sell it for the $100, list it on a domain name auction (there are lots of these), and let the guy who emailed you know it's listed on the auction. Start it at $100 so you'll get at least that from him presumably, and see if it's worth more on the open market.

Also i think you were just unlucky on the Valentine's Day domain swipe. If the search engines did this as regular practice they would be out of business very quickly!

Good luck!
Paul