Atlus has done a pretty decent job in keeping all of the Shin Megami Tensei titles in print - you can find copies of Persona 3 and 4, Shin Megami Tensei Nocturne and Digital Devil Saga all for around $30-$40 on Amazon, keeping them from following down the path of a Suikoden II. Nevertheless, the one subseries whose prices has climbed has been the Devil Summoner: Raidou Kuzunoha series, the action-RPG offshot that takes place around the turn of the previous century. The first game had reached prices of up to $80, with the second game not far behind.
Well, no longer, because reprints have begun showing up at Amazon and other places, priced at $30 new or less. These include Devil Summoner: Raidou Kuzunoha versus The Soulless Army and Shin Megami Tensei: Devil Summoner 2: Raidou Kuzunoha versus King Abaddon. It's probably to help tie in with the release Shin Megami Tensei: Devil Summoner: Soul Hackers coming out in a few weeks, though from both a thematic and gameplay standout they have very little in common. But nonetheless, everyone interested in them should check them out now.
If you can only afford one, go for the second one. Both games share many identical assets, but the second one is far, far more polished.