My kids wear "uniforms" for public school, but the tops are just plain polo shirts and t-shirts (in elementary only) in the school colors. Bottoms can be denim, khaki or navy. My oldest is moving from elementary to middle school and had very few polos in elementary as he preferred the collar-less t-shirts, so I bought him about 5-6 from Lands End recently when they had a good sale. I think they were about $10 each? I also got a couple of new t-shirts for my younger son at the same time, $7. They both have plenty of denim and khaki shorts to last until October, when we usually buy jeans. A friend gave me a bunch of shirts for my youngest son, as they're moving and her son won't have to wear the uniform shirts anymore.
I just got my youngest a new pair of Nikes at the Nike outlet on clearance for $20. I have to get new sneakers for my oldest, too, but his will be more expensive because he wears men's shoes. I usually wind up spending $50-60 on his shoes.
I often buy the Sears jeans for my boys - they're hard on the knees and Sears will replace them in the same size under Kidvantage.