There is a chap who advertises his carpet cleaning business on Spire FM ( salisbury ) very regularly, so he must be doing ok from it because I would think its expensive.
I have thought of it a few times in the past, it would give you a lot of credibility and certainly would get your name known very quickly. It has been talked about on here before and the general feeling seems to be its not worth it. I can't help but think its got to be worth a go, but of course it will depend how much it is.
Not 210FM, that is certainly the biggest one in the area, but at least you won't have your advert in the middle of Late Night Love with Graham Torrington and the cheesy records he plays.