Remember that clowns usually hire out for a specific amount of time, and they often know to the minute how long it takes to do their program. Few clowns will hire out for less than an hour, so be sure you schedule them for at least that long. Some might even need longer.
If you're considering going outside the local area to hire a clown, keep in mind the fact that many clowns (like other entertainers) may charge a travel fee. This is intended to offset mileage and wear-and-tear on vehicles and equipment, and shouldn't amount to much.