I would not recommend radio ads. They cost as much as TV ads but are less effective. If you were hell bent on radio ads, I would suggest your ads to be around midnight to 6am. Expenses would be much less
Radio is not near as expensive as TV, not even close. If you are paying anywhere near TV commercial prices for radio you are getting ripped off. And playing radio ads that no one in your demographics will hear is a complete waste of money. What market are you trying to hit by running ads from midnight to 6am. From 5am to 9am is great since it is drive time and when people are getting up getting dressed for work and listening to the radio. I have done radio advertising in many many many cities around the country. In reality I don't like radio advertising. Especially now with people listening to their Mp3 players and CD's. Back in the 80's it was okay. I would get more airtime and response when I did interviews on air. I have done a ton of them all over the country. When you go buy advertising the station will show you their Arbitron ratings to try and convince you to go with them instead of a competitor. Ads that run late at night are generally free. Radio stations have a lot more margin to play with pricing and airtime cause production costs are near nothing especially compared to TV. A radio ad can literally be done in less than 5 minutes and thats with writing and recording included.
If you are promoting an event you can have the station do giveaways on air to get you some free airtime. Donate tickets. A lot of times a station will match your dollars. So if you buy $2,000 in ads they will give you an additional $2,000 dollars in ads. The freebies will generally be spread out during the non-peak hours. If you want to get some good TV time, schedule an on-air interview during the morning shows or prime-time news. Every major market has these. Same with the newspaper. Call the paper up and tell them what you are doing and get an article done on what you are doing.
Marketing and Advertising is the most important aspect of creating a successful business. Hiring a big PR/Advertising firm will end up paying for itself very shortly. Just to give you an idea, I have budgeted an Ad/PR campaign for $250,000 for the first year of my business. That does not include promotional give-aways or in-house promotions. And, it looks like I may be raising that amount. The idea is to get your name out there in a manner that hits your target demographics. Don't waste your time and money on worthless crap.