When he thinks that he "deserves" that much car, ask him what you and the kids deserve. Also, is he a religious man? if he is, you can use the idea, that the man is supposed to listen to the counsel of his wife in all things and to put her before everything but God. So, if by buying a winter car, and keeping his car now, and YOU want a new house, which one is he putting first. My parents are very religious, well my dad mostly, and this is a trick my mom learned to help "persuade" him when he does less than brilliant things. Luckily, he has never wanted to spend that much on anything like that, I think the biggest was his Quad for like $9000.
Also, maybe you need to show him Edmunds, Kelly Blue Book, and NADA, and the prices on all 3 for age and mileage. Each one is going to be a little different, and banks, loan places, and dealerships all use different ones. Also compare private party and dealer prices!
Finally, look at Craigslist for a car similar to his and see what they sell for. Then, look and see if you can find one like the Audi he wants. I am assuming that he is going to buy from a dealership instead of private party right? The only thing that does is cost more money. They still don't give you a warranty or any guarantee unless it is new, and then that comes from the maker.
I also do not like car payments, but I do understand that sometimes they are needed, but not $30,000 worth. I was thinking maybe 3 or 4000 tops! I guess I am too poor to think that a car worth that much is a good investment. Oh, wait, I worked for GMAC for 3 years, and I know, its not a good investment.
Good luck, but really, let him see the numbers and see what his "idea" will cost him, and you as a family whole. Good luck!