I see no value to making a bar mitzvah expensive. the party is not relevent to the occasion and it is money that could be better spent, for sure. If you make it fancy and expensive by doing something to honor the fact that this is a day when a boy becomes responsible for Torah and Mitzvos, than that is something else.
We made a dessert buffet instead of a meal and we had it in a place where we could bring in food so our friends could bake for it. It was still ‘fancy’ because I have friends and kids who know how to make anything look beautiful but it was far from expensive. Better to save the money for the suit, hat, tefillin, hat, shirt, shoes (you get the picture)