I used to be a major player in Radio advertising, so I've seen the process from the other side of the fence.
The first thing you need to understand is the person pulling your quotes together has a sales target to hit, and therefore is going to present any information to you about their station in such a way that it looks like the advertising can't possibly fail. All they are interested in is getting your money into this month's figures... whether it's in your best interest or not. This is the reason it's best to cut an advertising deal at the end of the month when you can use the salesman's fear of not hitting their target to drive down the rate. Most advertising is based on a cost per thousand basis... there is a lot of room for negotiation on that rate... and nothing scares a salesman like a new client asking for the "cost per thousand rate"
The next thing you need to understand is that a radio stations figures are pretty meaningless, unless your product has universal appeal to a particular demographic. So, it doesn't matter that 40,000 people will hear your spot 6 times... it also doesn't matter how many times people hear your radio spot. Despite how many times a radio sales person says to you "Repetition builds reputation" it isn't true...unless you're establishing a local brand and then it does... In fact, if a radio sales person ever says to you "Repetition builds reputation" just look them in the eyes and say "And how is Giff Gifford these days?" when they've finished having a stroke on your carpet suggest that they cut the BS and talk to you like an adult.
Radio spots work on a very simple formula... does what you're offering fill a need or desire of the person listening?
Basically... WHAT'S THE OFFER?
If you've got a great offer then your ads will work... if you haven't, then they won't.
So, before you invest in radio spots (or any kind of advertising) you have to figure out whether what you're offering is going to motivate people to do whatever you're asking them to do.
The more effort the punter has to make, the greater the motivation has to be.
So it takes more to get someone to go to a cinema than it does to get them to buy something online, because you have to put your pants on to do the former.
The hardest thing for most businesses to do is objectively view what they are offering... the truth is advertising works, but most businesses don't. In all the years I worked in advertising, I would say 95% of the time business owners couldn't tell me why people should buy from them rather than their competition, without descending into meaningless cliches... like "quality" and "value for money"