I just went to an intro level Show and I think having the "correct" dressage attire makes a great overall statement about you and your horse. All of my dressage attire cost me under 30 bucks. I was given white breeches, I bought a white sleeveless show shirt for 15 bucks from a friend, my stock pin was a few dollars as were the hair nets. I bought my "show coat" from goodwill! Go find a nice tailored blazer for 2 bucks and add an extra button! SO easy, and cheap! I also wore white gloves, not black. I'm not sure which is technically correct. I wore paddock boots with black leather half chaps and they looked like tall boots!
Good Luck, and hope this helped!