Most of my stations pay a flat rate per month for stagers and ID's. Some of them only use me as another voice for the occasional tag, promo or commercial. At that point it's a different deal altogether because I'm not giving market exclusivity.
A brief search of the web shows
this non-union rate sheet (or the
pdf here) from Voices.com that may help as a template. But, you have to use your good sense to judge whether those rates will price you out of your market.