Whichever way you go, it's going to continue to go up in value. So in the back of your mind, you can rest assured that if you ever HAVE to sell it, you'll get back more than what you paid for it. Personally, I haven't ever used a guide to play a Japanese game. I think I eventually will though, for Ryo Ga Gotoku Kenzan!
I was able to get Saga in a bundle with the first two Panzer Dragoons and a Saturn 3D controller for $330 last year, all in very good condition -- actually the 3D controller had never been used. (I'm not a huge stickler about getting things mint, but if possible I like them to be nice.) So, probably about average, or maybe just slightly lower than average price, depending on how much you value each item. I'm hoping to play Saga some time this year, but there's an unusual problem with my Saturn's memory so I need to either replace it or figure out something else to be confident that I won't get way into it and then lose the saves.