My kids attended Catholic schools from K-9th grade for my daughter, and K-4th grade for my son.
My daughter's uniforms in particular, were expensive. She had to have specific plaid jumpers and later, skirts, that could only be purchased at a local Catholic school uniform shop. Along with particular vests and sweaters from the same shop.
At the Catholic school she attended from 6th-8th grade, the students had to wear school logo polos available only at the uniform shop. At $30 a piece. Yes, literally, $30. That was 10 years ago. I cringe to think of what they likely cost now. The girls were also only allowed to wear white polos, vs the tan or navy blue the boys wore. Those white polos inevitably ended up stained beyond repair by the end of the school year, and then had to be thrown away.
Add on the specific types and colors of socks and tights the girls were allowed to wear. Then of course, gym uniforms, allowable school shoes, school sweatshirts, etc.
It got pretty expensive fast, and cost me more than just buying clothes that can be worn at both school and home, as when we switched to public schools.