Try to budget as much as you can afford for hiring your clown. Be aware that you can usually hire a clown for almost any (reasonable) budget, but you'll be hit with the truism that you get what you pay for. You won't be getting Clarabel or Bozo for $100.
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.