When you enter an audition, a Casting Director wants to see the character and hope you are the one. So make their job easier... Look the part --- ish.
No need to spend a lot of money. Playing a Lawyer? Wear a collared shirt. Playing millitary? Get a khaki T-shirt and 50 cent plastic dog tags. Spend $2-$10 at a thrift store and look like you're in the wheelhouse of the role you're gonna play.