Average here is $30-40 / horse for a trim.
I pay $35 and my gelding is trimmed about every 4 weeks during the summer and every 6 weeks in the winter. I let his hooves tell me how often he needs to be trimmed. It takes her about a half hour to trim him. I stick to a very strict schedule because he is likely to go lame if I don't and its not something I'm willing to chance (previously diagnosed with "navicular" as the result of bad farrier work).
Every horse is different and will need a different schedule depending on certain factors.