The purpose of a traditional wedding is for a man and woman to pledge their life to each other and to make it their mission to help the other reach Heaven. The point is to stand up in front of their community, friends, family, God and take those vows. We invite the people we love and respect to our weddings so that the promises made are more serious. The audience is there to witness the vows and to hold the bride and groom accountable for the words they speak that day.
Of course many weddings have moved away from this serious thing into selfish, secular nonsense. It's turned into a birthday party for grown women. Spouses don't take their vows seriously anymore. Many people consider divorce to be a valid acceptable outcome from a marraige, instead of a shame on all involved.
As far as the cost, that should be the least of anyone's concerns, when you are dealing with pledging your life to someone. Love of God and love of family should be so far above and beyond your concerns with money. Besides, traditionally the brides' family pays for the wedding. That has been the tradition for centuries