The DNS hosting service we used in the past charged us $13.95/year per zone.
Slicehost doesn't charge you anything for DNS hosting if you use them for some kind of VPS hosting. We use their DNS for our (dev and staging) servers that aren't even hosted on Slicehost and it doesn't seem to be a problem. It's the one reason why we continue to keep a few of our VPS servers hosted there since competitors such as Linode are so much cheaper now.
Slicehost doesn't charge you anything for DNS hosting if you use them for some kind of VPS hosting. We use their DNS for our (dev and staging) servers that aren't even hosted on Slicehost and it doesn't seem to be a problem. It's the one reason why we continue to keep a few of our VPS servers hosted there since competitors such as Linode are so much cheaper now.