Cost shouldn't even be a consideration. Because we expect our advertising to PAY for itself. Remember, it's an investment not an expense. One ad rarely sparks a buying decision. Usually we must be exposed to several ads over a period of time unless we have an immediate need and we just happen to hear the commercial at the right time.
Advertising build customers over periods of time.
Radio should not be purchased in spots. Instead buy blocks of time. Your commercial played several times over a short period of time is better than your commercial played fewer times over a long period of time. Buy a block of 42 commercials. These would be played on Tuesday, Wednesday and Thursday.