I am interested to find out how much you can charge for an audio
advertisment if someone were to be placed on hold via a telephone.  In
example, lets say I call Moviefone and I hear a 30 second ad, how can
I figure out the advertising rates for this?  I understand that the
larger your audience the more you can charge, but is there a rough
estimate to say that you could charge an advertiser say $1.00 per 30
second ad or could it be more/less?

I am working on a business plan for a school project and am trying to
understand the ad model here.

Thanks

I am not sure if this helps but i found a page in some other language. 

http://www.ctimail.com/eadvertising.html

The cost of 30 sec ad for 1000 Ads is from  $255 to $500.
So, cost per one 30 Sec ads is around 25c to 50c. 

If you have found more information i would be interested to know.