I'm paying $450 for mine (hemming, bustle, cups added).
It is expensive, but you also have to consider the time and labor that goes into altering a gown. I thought my alterations wouldn't be very expensive, but I wasn't aware of the amount of work that went into the hemming (a complicated seam with very thick fabric).
If you think about it, would you really trust your local dry cleaner for your alterations? Sure, they'd probably be a LOT less, but who knows what you'd end up with.
I'm ok with paying for my alterations because I know what I'll be getting in the end. I'm 100% confident in my seamstress (she's an old Italian lady that worked with Neimans and Chanel for over 20 years).
Is it more than I expected to pay? Yes.
Is it worth the peace of mind? Definitely!