lesschumras: There are no expenses requires for a Bar Mitzvah, nor for a Bas Mitzvah. Almost any shul will give the boy an aliya, and if you do it on Monday or Thursday they never expect a donation. Actually, the aliyah is just a way of showing off that “I am now an adult”. Suppose he did something else like enter into a contract to show he had reached adulthood– not a good idea. You become Bar MItzvah automatically on your birthday – no ceremony required.
Ah, you like to spend lots of money on parties — well that’s a different matter and is totally unrelated to the halachos of Bnei Mitzvah.