Even though it's the best-selling GC game, most owners aren't willing to sell it because it's got high replay and collectibility value, and many people want to buy it (like you).
According to Wikipedia, Mario Kart: Double Dash!! is the second best-selling GC game and sold almost as many copies as SSBM, but you can buy it for a more reasonable price.
Luckily I already have a copy of SSBM, but I'll probably face the same problem with other great titles I don't have, like Wind Waker or Twilight Princess.
TL;DR: number of copies sold aren't a good way to estimate a game's price.