700 guests? Does your wife/her family really know that many people? That's a lot of 'friends' to have at your wedding. Catering can go to about 200 baht per head (that's 140,000 baht). Sang Som is not that expensive, and the fact that most Thais will dillute it with ice, they're not going to drink that much. Same goes for the beer. Does the 500,000 baht include gold for your wife and her family? If it's just for the party I'd do a bit of maths first...
Is this in a hotel, or a local plastic chair and round table brigade? The latter would be much cheaper.
But if you love her, and you're able and willing to pay, just go with the flow...
VS