So due to the ER I am now addicted to SMT games.  Would I be insane if I paid $130 for Digital Devil Saga 2?  Should I save my copy of Persona 4 like a treasury bond and hope that everyone else sells their copy to gamestop?

The only reason why SMT games tend to command high prices is because of a demand issue, as most of the ones released outside of Japan have had a pretty healthy supply. Nocturne and Persona 3, for example, have had multiple print runs in order to satiate customer demand and calm the market down. As such, like a lot of such highly coveted games, market prices tend to fluctuate for SMT games, which is why I'd just recommend keeping an eye on DDS2 prices and wait for them to fall for now. I guarantee you that I've seen copies of that game in particular go for far, far less, even right now. Because of that, I personally don't see Persona 4 being worth much more than its current retail price because Atlus has actually managed to make supply keep up with demand for once upon  its release. The only way you could make the game more valuable is if you bundled not only the artbook and soundtrack, but also the Amazon Social Link Expansion kit that was up for grabs a few months before release. But in that scenario, you probably wouldn't be making much of a profit at all. At the very least, Persona 4 is probably never going to repeat the scenario of Persona 3's original release. Too many people know about it this time around and I think most of them have already gotten their copies so that they don't have to deal with Atlus' syndrome of having the game disappear from shelves almost instantaneously because of a low print run.

Persona 4's printrun was much, much larger than DD2.  It sold something like 250K in the US.  As for paying $130 for DD2, every so often it seems like Atlus fnds some old boxes lying around and new copies sell for list on Amazon.  And Atlus has done newer print runs of Nocturne.  Right now the first DD is on sale for $35 at Amazon.

Hmm...wonder how much my original P3 (non-FES version) goes for these days...
Not that I would ever sell it.  I <3 my Persona games far too much!