While the spending listed in the article is truly ridiculous for a high school dance, it's not really a meaningful average. From the graphic at the top of the article:
"The averages listed are compiled from a survey of 700 readers of Seventeen Prom and TeenPROM magazines last year, as well as additional USA TODAY research."
I don't know what additional research USA Today did, but the sample from the readers of the two teen prom magazines is almost guaranteed to be heavily biased toward the very highest spenders. Interestingly, there is a poll about prom spending on the article itself that looks more like what I would expect. Here were the values when I looked at it just now:
How much do you plan on spending on your teen's prom?
Nothing
24%
$1-$250
36%
$251-$500
24%
$501-$1,000
9%
$1,001-$1,500
2%
$1,501-$2,000
0%
$2,001-$2,500
0%
More than $2,500
5%