It depends on what type of show you are going to, what type of saddle/attire your horse is in and what you plan on doing in the future.
If you are doing w/t rail classes, have an all purpose or close contact saddle and plan on showing hunters and jumpers, then tan breeches and a navy coat will be good staples to purchase now. Spend the money and get good quality breeches, and a nice coat. There are many places selling Tailored Sportsmen breeches for under $100.
If you are planning on showing dressage in the future, then the purchase of a pair of white breeches and a black jacket will serve you better. Full seat breeches are preferred if this is the route you are taking. Again, you want to go for quality. You can see EVERYTHING under white breeches, so spend the money to get a good quality pair that aren't see through!! Both my show breeches were over $300, but you can get a nice pair for around $200 if you forfeit the leather seat.
Good luck!