Clowns, superheroes and mimes are overrated. There are easier and less expensive ways to entertain a group of kids. Movies, video games and puppet shows (for younger-skewing guest lists) are all easy to do on the cheap.
If your kid desperately wants to invite a costumed guest, you can always rent the outfit and have a friend dress up to save some money. Once your child is old enough, you can even ask for his input. Try explaining that Spiderman charges a handsome fee for appearances, which might mean skipping the goodie bags. He may not like having to choose, but your kid will feel empowered to help make decisions about his party.