I think a "fair price" sort of depends on who you're talking to. Some people think that 600-700 dollars is a "fair price" for certain "rare" CDs, but others would never pay that much for a CD, period, regardless of how "rare" it is. But, here are some things to consider.
Original price: How much was this originally? In this case, 4704 yen. That's basically $60 now, if you want to convert to USD.
Condition: This will basically affect the original price. If it's still brand new, it's perfectly sensible to ask for the original price, since that was the price it was sold for brand new. If it's used, you shouldn't charge as much as when it was new.
Rarity: This is the bullshit factor that people jack up the price into the hundreds for. However, if a CD is old/rare, it is fair to ask a little more for it, since it's harder to find. However, even limited edition enclosure soundtracks dating back to over 20 years ago are still only going for 500 yen. I bought Cosmic Psycho 8 months ago for 500 yen in great condition, and it's
still going for that much today, so if something that obscure is only going for 500 yen,
a fair price isn't going to charge excessive amounts of money just because something is "rare".
Shipping & Handling: How much will it cost to ship to your buyer? This one's pretty obvious.
That's what I would base things off of, anyway. If you want a fair price, it shouldn't be about how much money you
can get for it, because you can probably get a lot. If you want some great prices, look at people like the_miker, Xenofan 29A and Chudah to get an idea of how sensible game soundtracks can be.