I think The Flash is right. It's not quite the same, but I've been looking for a copy of Colloseum or XD for a while now. I was certain I could get a cheap one, but no matter where I look they are still release-price or more. It is most likely because the games are still quite popular because of the unique game-only Pokémon, and the fact they are getting "rarer", so they can argue that prices can be high. I had a similar problem trying to buy a GCN copy of Pikmin 2; the price was going up (untill the Wii version came out).
Therefore, because people still want Emerald for the RS Pokémon (e.g. Legends) and Pal-Park stuff, plus GBA games are getting thinner on the ground, then a more premium price can be charged compared to the usual GBA bargain-bin tat.
I reckon the price of Emerald will drop eventually, either when HG/SS come out, or when the DSi gets to a point where the GBA slot is considerably more phased out.