I'm a sound engineer, and we do radio! We can do spots for anyone, anywhere. Our rates are $225/hour, and if your spot is simple, it could be done in an hour, but you would have to add the cost of the voice talent and music licensing fees.
What you're probably hearing are spots created by the individual radio stations. Call their advertising rep and they'll give you a quote that includes airtime and the actual production. They'll use their own DJ's as talent, and they already have music licenses in place. Pick your station and give them a call!
For 3 days, I get to part of a thousand other journeys.