Because of the ROM and the fact that Chrono Trigger carts are no longer in production, the cart has become obsolete for anyone other than those who prefer a cartidge copy over a ROM image copy, for whatever reasons, and those who collect appealing carts as much as a numismatist collects appealing coins. These two groups of people--the ones who prefer the CT cart to the ROM, and those who value the CT cart not only for being CT but also for being a cart--are the ones who determine the "value" of a given CT cart today. What they are willing to pay, on average, represents what the carts are still worth.
If we could sample these two populations it might be possible to determine with some confidence an empirical average value of a CT cartridge in various conditions. So if you're interested in answering this question Fo' Real, that's where you'd go with it. Just my $0.02...which, incidentally, is more than I'd personally be willing to pay for a CT cart.