Arbitron provides the measures of listeners used by the industry to get ad pricing.  Changes in the tool will change buying practices.  This article on the Arbitron® web site provides information about the Portable People Meterâ„¢ and the impact on Planning & Buying Radio Advertising in a PPMâ„¢ World

http://www.arbitron.com/downloads/arbitron2008mediaplan.pdf

Strategic Media, Inc

Site with a variety of short pieces on radio, radio advertising and "how to's'" 

http://www.strategicmediainc.com/radio-advertising.php

Businesstown.Com

Provides an overview of planning and implementing a radio ad aimed at small businesses including The Lowdown on Radio Advertising, You're Not Buying A commercial; You're Buying an Audience and Writing Radio Copy.

http://www.businesstown.com/advertising/radio.asp

Radio Ad Lab

A report issued in August 2008 provides insights into the values for  quality radio advertising

http://www.radioadlab.org/studyDocs/engagement2Full.pdf

Burkhard Works

A compendium of resource materials borrowed from other sources in one place. 

http://www.burkhardworks.com/radioshow/advancedconcepts.htm

Ad Buying Secrets

An internet site that offers an insider's guide to buying radio and TV advertising for small businesses
http://www.ad-buying-secrets.com/radio-advertising-price.html