Wow...............I'm speechless.
I have twin girls going to junior/senior prom this year. My limit is about $100 for a dress - give or take a few $'s (I'll go to about $120 if they completely fall in love with the dress).
Here's our cost so far:
$99 + tax for dress = $108 total
$00 for shoes & garter for one DD with a gift certificate someone gave us
$50 for limo that I said I would pay
$00 deal is she'll pay for her own hair
$25 for prom pics for her & date
$3 for boutteniere (sp?)
$00 for make-up. She has her own & will do her own (older sister may help)
DD #1 = $185
$25 for dress because DH promised each of them $25 if they found a dress out of their sister's closet!!!

$00 for shoes - using ones we bought for homecoming
$45 for limo (they're in separate limos

)
$00 deal is she'll pay for her own hair
$25 for prom pics for her & date
$3 for boutteniere for her date
$00 for make-up - works the same for her as it does for sis
DD #2 - $98
$30 for dress alterations for both of them.
I will probably have to get charms for the garters - $10 each maybe.
My cost is about $333 in & that is more than enough for me. I'm sure there will be little incidentals. We'll probably give them money to go out to eat with after the prom or something, but seriously, it's prom, not a wedding.
I even think some of the things the OP priced out are ridiculous for a wedding, but that's JMO.
Like I said - I'm speechless.