Media Buying is easy, doing it right is hard.
People call my office in San Diego regarding media buying all the time and ask something like "How much does advertising cost?" My first answer is usually something like, "How much does a house cost? It depends on how many bedrooms, square feet, stories, location, swimming pool, etc."
Buying advertising is like building a house. There is a lot that goes into a good house. There are plenty of important decisions to be made long before you hammer the first nail. You should never design the house before you choose the lot on which it will be built.
It is the same with advertising. You should choose the medium through which you plan to advertise and only then write, design and produce the advertisement. If you have a radio script in mind before you start your research to choose a medium, you will be prejudiced to find reasons to select radio.
The first decision that needs to be made is how much cash do I have available to spend. Never buy advertising on credit your first year or two in business. Any return on your advertising investment is likely to come after your payments are due anyway. Credit in advertising can cause a cash flow nightmare. Instead, choose the correct medium; outline a reasonable budget, negotiate wisely, understand every conceivable expense before signing any agreements, and then prepay the bills (securing any additional "pre-pay" or "net ten day" discounts.)
If you have a large advertising budget you have many choices of what type of advertising to do, including broadcast (radio and/or television), one station or several. If you cannot afford radio or television your choices of where to advertise are reduced. Maybe you should choose to start in newspaper and/or direct mail. Even with a large budget newspaper and/or direct mail may be your best bet. It depends on your product or service.
Do not spend a lot of time researching a medium you know you cannot afford.