Wedding budget help!

My step daughter wants to have a grand 'weekend in the country' wedding party. We live in the United States so there is quite a bit to learn before booking! My husband and I gave her what we could afford as a budget; her mother and spouse will provide the balance. My issue is said mother-of-bride is hell bent on having the wedding of the decade I believe and thinks we should set a budget of a MINIMUM of £200K, which I think is quite insane. We reiterated our budget and as their partnership is very wealthy and quite able to afford whatever they want, told them they are welcome to spend whatever they want but we will not be matching them. Without getting into issues, my question is this: what is a reasonable budget to set for a 3 day weekend, sporting activities and rehearsals and dinner day one, wedding day 2, relaxation day 3. She wants to rent a manor house for approx 20-30 guests and wedding of 70-80 people. Where shall we start?!!