You NEVER lift your arms up in a strapless dress! Your boobs slip out of the dress!
Also, those dresses are expensive.....I guess showing off to the one schmoe in the reception is worth ruining a 1k+ dress
Nah, most are heavily overpriced. If it's real mulberry silk, with good quality lace and/or hand sewn (real) pearls or semi gems, yes it could cost a few thousands of dollars/euros. Some of it is for material, some for the seamstresses, most for the idea that you really have an expensive dress...

(my aunt was a seamstress for wedding dresses and suits)
You could buy a dress that looks nice on pics, but is made of polyester and sewn with cheap plastic 'pearls' or 'rhinestones', made in China for under hundred dollar. Perfect for doing stunts like this.