Just because a person has a wig and makeup doesn't mean that they're a good clown. Before hiring a clown, ask them about their education and experience. Otherwise, you're likely to get someone who just happens to fit into the booking company's clown suit.
Be realistic when hiring a clown. Top entertainers -- especially children's entertainers -- can draw in as much as $300 or more for an hour's performance. If your party budget can't stand that kind of blow, you might want to consider another form of entertainment.