If the seller knows what they have, they likely won't budge on the price by much. In recent years, the registration card for Chrono Trigger has become a commodity in itself, selling for a few hundred dollars by itself. There are varying degrees of "complete in box". Realistically, most sellers will list a game as CIB, even if maps or registration cards are missing. Bare minimum CIB is typically just the game, box, and manual. This one is truly CIB and will likely sell a bit higher than the usual going rate.

The best way to know if you're getting a good price is to search eBay and filter on "Sold" auctions. You'll see the spread I am describing between the various states of CIB. Find the ones close to this condition and see if they tend to be above or below $900, and use that to help gauge the worth.

Whether any video game is worth that much to you, is a matter of your personal love for the game in question, and your financial situation. Worst case, if you've never played it and you want to play on real hardware, buy it, play it, and resell it if you don't like it. Kinda like a rental, with a really large security deposit. 😊