I think you just need a clear outline of what you want and a number you are unwilling to go above. Let her know what your "must haves" are and the budget it needs to happen within. It's their job to figure out how to make that work. Be prepared to make concessions in the areas of less importance to you. If you can be flexible in the time of year, you'll have more negotiating power. I think you also need to outline the type of menu you're interested in. If you have time, research venues in the area and try to find a comparable venue for a lower price. If you can present lower numbers, this also increases your bargaining power. Don't go in acting as though money is no object. I see so many people doing this, thinking it makes them sound "classy". Classy people have budgets. Let her know your primary concerns(ours were quality, elegance, and COST).
Try and walk in with a clear direction of these things:
Tone of the event(modern, rustic, themed, etc)
Food(passed or stationed hors d'oeuvres, number of courses, type of menu, number of offerings)
Your personal responsibilites(the things you will take care of and needn't be in the quote)
Time of day
Number of guests
Tableware(chargers, full china, toasting flutes, etc)
These are the biggest things to establish right off the bat. Pretty mych everything else can be decided in subsequent meetings. Try and negotiate all you can before signing the contract. Hope that helps :)