This question is much like asking, "how long is a piece of string". I find the price per hour is totally irrelevant, you have to account for the time it is going to take for the developer to complete the task.
For example who is cheaper, the $10 per/hr developer or the $50 per/hr developer?
Generally the better developer charges a higher rate but can produce the end product much more competently and faster. It may only take the $50 per/hr developer 2 hours to do the task, but the cheaper $10 per/hr developer takes 12 hours to do the job.
Personally, I would rather pay a developer by the job instead of by the hour. For mission critical applications, I would also pay a little more and hire a few developers to work together on the project. The more people who see the code the more likely someone is to spot a bug or security hole. The complexity of the script and the language in which it is written also come in to consideration. There are too many variables to come up with a single, fully correct answer to this question.