I would rather pay hourly because the amount of changes / iterations that are going to happen. I'm a programmer and I know hard estimating is and it seems weird

I would rather pay hourly because the amount of changes / iterations that are going to happen. I'm a programmer and I know hard estimating is and it seems weird to charge '30%' up front... What is 30%? What if our iterations go longer the designer though? Do I get billed more even though I paid up front?