Real cosplay is not that cheap, the cheapest you can get is around the 1k-2k range. Most of the price is from people making it by hand, the materials used, measurements, and whatever else.
$300-$500 is the tell of an 'outfit from china' price. And that stuff will easily damage itself if anyone looks at it wrong, or be a scam and you'll never get the outfit. Trying to get something from China should be an absolute last resort.
And with her being an XL, she would look like a trash bag dressed up in a blue dress and will still look ugly anyway. Props for having confidence even if it was misplaced, I guess?