I own the game. Complete. Saying that, this is one of those games that I think the price is completely ridiculous on.
There were an estimated 150,000 copies of this game made. It's not rare, when compared to a lot of other SNES games. Uniracers had less than 300,000 copies WORLDWIDE, and it sells for like $10.
I can understand complete copies being rarer than other games, as the box was huge and if you weren't a big fan, there was no reason to keep it around.
That being said, it's my understanding that anything under $100 is a decent deal these days. Also, Earthbound is my favorite single game of all time (my old avatar was a starman from the game), so please play and enjoy, even if it requires an emulator.
I attached my Earthbound/Mother collection, for gloating purposes. (Yes, I'm missing Mother 3. I'm someday going to drop the $400-500 for the Mother 3 gba. No reason to buy it twice
)