Limited experience, here's what I can say
Concurr doing 15 and 30 second ads. I've tried very limited radio twice with my current business and broke even both times. My market is about the #100 metro area, but at the time I was doing a mobile service (bringing samples to the homeowner).
If you are "producing" the ads yourself, i.e., not using an ad agency, you should get at least a 15% discount off their rate sheet. If you commit to a run of X number of ads or X months you should get a discount. If you use an ad agency you won't get the 15% discount, but they should do all the work and plan your campaign - in exchange they get the 15% as commission. In other words, you pay the regular rate (less any discount for longer contracts) but you don't pay the ad agency, except MAYBE a talent fee for recording the ad.
You can look at ratings for various stations, but the reps ought to show them to you, or your ad agency should. It should give demographics (age, etc.). Ad rates essentially reflect the listnership and demographics.
You will often, thru your rep or ad agency, get a "special" when they are trying to fill unsold time - they can't inventory it. Remnant time can be very cheap. At one company we ran ads on a reasonably popular country station that was as little as $2 per spot. Downside is you may not have a set time these ads run. May be 11 am, can be 3 am. You could find a remnant special and buy a lot of cheap remnants on all the stations the company has in your market.
REPEAT REPEAT REPEAT. My budget won't let me do enough ads, so I have been trying other methods. But the reason you hear a Lowe's or HD ad every hour on every station is to saturate the market so when the aveerage person is in the market for something they have they are the only place that comes to mind to go look. Your job is to get people to automatically think of you when in the pool services market.
Read "guerilla marketing' thread re: repeating ads. Marketing people say people need to absorb your name minimum 3 times for it to stick, but we only notice 1/9 of what we see/hear, so that's 27 exposures needed. But people will only hear a portion of your ads, so if someone listens to your station(s) and hears 1 of 10, that's 270 spots needed. And to get as many people as possible it means running on lots of stations. Say 5 stations, and if the average person hears 1 of every 20 ads, that's 540 spots x 5 stations or 2,700 spots before you should start to get anything like a real measure of what it might can do for you.
Have your accountant compute a break even analysis. Basically "cost of ad campaign" divided by gross margin per average job = number of jobs you need to pay for it. See if that is reasonable expectations.
Good luck, and I'd be interested in results you get.