Reach out to millions
Google has a service called "Google Radio Ads". They strategically purchase all "dead" air-time from the major radio stations and resell it. Compared to the traditional radio advertising methods, Google's service is very inexpensive. Small campaigns run anywhere between $100 to $300 per week. Most traditional radio ads run north of $800 per 30 second slot.
Target Customers with Radio Ads
You will have the ability to target your customers based on geographic location, or select different station types, demographics, days in the week, and times of the day.
Effectiveness
Every situation is unique. For some, radio is great for branding. For others, radio will increase traffic and conversions.