I’d offer what you want to pay for it. If you miss is for the sake of £50 for instance, would you be gutted and keep revisiting it and kicking yourself? It often depends on what something means to me as to how much I bid.
I'd say about 40% but this depends on dry cleaning as that can be bloody expensive for wedding dresses. My heavily beaded (this means mega cleaning bucks) wedding dress is still covered in mud and has been in the cupboard for over three years now . I bought mine second hand for about a third of the price but it was a sample and I spent about 15 hours sewing beads back on.