Hi everyone! This is my first post on The Knot-so nice to "meet" you all

I had my first trying on dresses experience yesterday at a small bridal shop in my city. I tried on one that was PERFECT...it is really beautiful and I look great in it! Not to mention it is perfect in size-the only alteration I will need is to put molded cups in the bust and a bustle.
The only problem is that it is more than I want to spend. I am on a VERY VERY tight budget and I really wanted to try and find a dress for under $250. (I have found plenty online but I'm a little nervous about doing that because I won't be able to see how it lies on my body). So this dress is $500-which is a very good deal for this dress in the first place, she has discontinued it at her shop. I'm wondering how much room is there to negotiate at a small, independent bridal shop? Would it be a total insult if I offered her $400? What if I said "If I make an appointment to have the alterations done here will you take $400 for the dress?"
Sorry this turned out to be kind of long-any help will be much appreciated!!