I've noticed that Chrono Trigger sells for a lot of money on Ebay and several of the listings claim that the game is very rare. Sometimes up to $40 for the cartridge alone. However, I recently saw a rarity guide on Super Nintendo games that rates Chrono Trigger as a D rating for rarity.
http://www.invertedweb.com/community/article.php?a=371
The list was compiled using auction sites to discover how often a game is actually listed. I've talked about this on IGN and some other message boards. Some users actually say it's not rare at all. They actually produced 2 million copies in the US. The amount of people listed on Gamespot that actually own the game is pretty high as well.