It's not as black and white as "it works or it doesn't" - uniforms probably won't transform your son's academic performance, but they're not pure pointless spending either. The real argument isn't so much about changing behavior (that's more of a myth), but that they reduce visible inequality in the classroom, simplify mornings without having to pick out clothes every day, and some parents value that more than the cost.
That said, your complaint about the money is legitimate. The prices schools charge for uniforms tend to be inflated, and with kids growing every semester it ends up being a pretty annoying recurring expense. If most parents in your group feel the same way, maybe it's worth putting in a collective complaint to the administration - there are places where they negotiate more reasonable prices or let you buy from other suppliers. What you shouldn't fall into is the idea that without uniforms everything falls apart; it's not magic either.