I just searched for the game on eBay, and it's going for upwards of $1000 AUD, including postage. Sellers are describing it as "rare", and it looks to be an accurate description in this case. I've found a couple of articles talking about it. The
first says this:
So, why is it so rare? This game is one that didn't receive great reviews, especially the GameCube version. Because of that, there were likely not a lot of copies sold. This means that a new copy of the game can cost $399 or more.
The reviews indeed weren't great, so it is very likely that there aren't that many sold copies now in circulation on the second-hand market.
Another article notes that its price has increased over time, as people became aware of it as a rare collector's item:
In our our earlier guides, this game barely registered despite its rarity. You could easily find Disney Sports Basketball for between $10 and $30 through 2015. The collecting crowd finally caught on in 2017, when it started hitting its current range.
That suggests the cheap copies have been bought up, and as people have realised they've got a rare game and sought after collectible on their hands, the prices have continued to increase.