I've also noticed this happening on some DVDs I had in my "save for later" list on Amazon, some went from 10$ to over 100$.
Honestly, I think one of the biggest factors is Ebay's free listings policy that allows sellers to put up games at insane prices without having to pay a fee on their asking price. When prices get pushed up, everyone seemingly wins: Ebay gets their fees; resellers make more money and can make a living from relentlessly scrounging at yard sales and flea markets; collectors feel important and like they're making an "investment"; the press benefits from playing up the hype...
...everyone, that is, except gamers on a budget. And casual collectors, too -- the kind of people who'd like to have a nice library of games without paying retail prices for each title (or higher). But hey, at least we've got flashcarts. I'm just glad most Genesis games are still cheap!
I suspect the prices will moderate over time. Some buyers may see the rapidly increasing prices and mistakenly think "I better buy now before it gets even further out of reach". It's like the housing market of a few years back - people were frantically buying overpriced houses.
The time to get a good collection of games from e-bay was about ten years ago. I remember getting some good deals back then. Now the prices are very high! I was going to get a Fair Child Channel F system a while back, but I put it off. Now they are $250 to $500.