It's very tricky to cost the alteration without seeing the dress. The cost will depend upon the layers in the dress and the fullness of the hem.
In most case the front of a bridal gown only needs to be shortened as a bustle can be added to the train of the dress to make it more comfortable to wear.
Lace wedding dresses can sometimes be more complex and expensive that's why it's good to see the dress. An average hem alteration and addition of a bustle would be $200 approximately.