Unreasonable dress budget?

Will it be completely impossible for me to find my wedding gown and have it altered, all for under $850? This is what I’ve budgeted, and I really don’t feel good about spending much more on something I’ll wear once. I’ve had multiple friends laugh at the idea, saying alterations alone will be $500 and I’ll never be able to do it. FH and I are having a fairly expensive wedding (average I think, for our area, but not a small budget by any means), but we’re foodies and love a good party, and so we decided to dedicate the bulk of our budget to an amazing caterer and premium open bar. We agreed that things like my gown, flowers, decor are lower priority in the budget (and I already found an amazingly low priced florist with great reviews).

I guess I’m just looking for encouragement from ladies that found their dream dress on a budget (and recommendations for where). AND for someone to tell me that I’m not doing things completely wrong...😕