I appreciate you not feeling defensive and understanding that I am trying to help you.
Would it be ok to ask what your product is? I mean, you don't have to give specific info but are we talking self-help DVD's, or some kitchen gadget, or supplement of some sort?
You've obviously looked into the cost of the advertising as you were very specific about what sort you would do. May I ask what lead you to choose this advertising option?
I am also assuming that you have an online store, e-merchant account set up and functional? You also got the business license in order (ie LLC or sole propietor, etc... benefits/drawbacks to each). Depending on the product I am assuming that you've got the numbers in front of you as to how much of the product you need to buy from the manufacterur (size of batch), price for corresponding number of containers, labels (stick on or laser), if you need any sort of licensing/FDA approval, etc, what sort of liability insurance do you have (I assume you've got all of this factored into overhead), where will you store this stuff until it is ready to be shipped? Is your product subject to spoilage? Is your mark-up enough to make a profit after operational overhead and other such expenses? Is it too high that your product won't sell out of the starting gate?
And what will you do to stimulate business if the radio advertising doesn't do what you think it will do initially?
So many minute variables that peeps don't realize goes into such a venture until they are balls deep...