The amount you spend on a set of spurs will depend on the type of spurs you want and the quality of the spurs. A simple set of spurs will be relative cheap, whereas an ornate set of Western spurs will cost a lot more money.
One thing to remember when buying spurs is that the cheapest option will not necessarily be the best. Firstly you need research what type of spurs to use, then find the best quality ones of that type that you can afford. A cheaper set of spurs will potentially rub the horse or be uncomfortable to wear.