So you are ready to go to a live action event and you are faced with a that classic quandary... what to wear. People often look to LARP and Sports Reenactment events and presume that the clothes they are wearing are costumes. While this is true in the most basic sense, the reality of the situation is far more complicated.
A costume is of course any covering that is designed to represent another thing. By that definition of course people in garb are in costume. But what we reference as garb stands apart with the distinction that it is designed for the level of events we participate in. Garb made for Sports Reenactment is reinforced and durable, able to take the beatings, wear and tear that Sports Reenactment will offer. Garb will use real buckles and straps and even real metal armor as needed.
Costumes take great shortcuts, as they are not intended to function in any way but just give the appearance of garb. Costume leather straps are often not actually leather, the rivets and buckles on costume grade pieces are not real or at best they are composed of thin, non-functional materials.
Cosplay on the third hand is often a hybrid of the two. Depending on the dedication of the cosplayer, the realism and durability of a piece can vary widely. Many cosplays are as close to authentic as any garb. While the foam armor may not hold up to combative arts, the sewing and leatherwork of some cosplay will.
A good kit serves the purposes of the wearer. For those who are not going to be putting much stress on their clothes and such, a costume is fine - although the step toward cosplay is appreciated over a store bought costume that looks clearly fake. That does not mean one must spend hundreds of dollars or more to get good gear. Craftsmen within the Kingdom can craft good garb for you at a reasonable price. They may even work out a good trade with you.
Remember, there is one good rule of thumb when it comes to how realistic your kit should be. If you are functional in it and at a range of five feet the kit looks nice - you have done a good job. By all means have fun with it!