different family has different tradition. It's not a "Must" for all the chinese tradition to be used in your case. If you can't afford or any reason, you can still buy other things for your future daughter in law.
My father bought a diamond necklace for my wife for the gift.
Remember, during the wedding, it's better to communicate with your son and future daughter in law, they may not like a big golden pig to be hang on the neck. It's not good or easy to match with other clothes but only the chinese wedding gown. Therefore, it may not be the case that the bride could wear the whole night.
Try something new or pretty, your son and daughter in law would like it more and they would be more appreciate if you think more on their side.