Al-Raheem, may I offer a suggestion?  Ask your wife and her parents how much they would pay to have peace, friendship and a harmonious relationship with the rest of your family.  Would it be worth $20,000?  Especially if wasn't even their money?

I agree, it's a terrible waste; but after all, you did promise, and I think your parents are being very generous by not expecting your in-laws to contribute a dime towards it.  (Or perhaps they wouldn't be expected to pay anyway.  I'm not sure how it works in your culture.)

Your parents want to have a party in your honour.  They're proud of you and they want to show you off to their friends.  It's a lavish compliment -- far too lavish, in fact, but try to accept it in the spirit in which it is offered.