Personally I believe this is a much wiser approach than dedicating full time people to quality system auditing. There are guidelines for this that they are obliged to follow, with lots of wiggle room. They are free to set their own pricing schedules per auditor day and other fees, though. And the quotes can vary considerably. The amount charged per auditor day isn't really very helpful information because it doesn't give you the whole picture.
Price is a factor but by no means the most important one.
There are some very inexpensive registration bodies who are cheap because they are not very good, their registration is not well respected, they do poor audits and provide lousy service. They are required to make that information available to you. Their business practices and policies. They are required to provide you with detailed information about how they operate. You need to feel comfortable with them.