Typical Commercial Costs for Providing Secondary DNS Service

[Jim]

I'm interested in having a secondary server in a physcially separate
location.  I was wondering how much commercial services charge to
provide secondary DNS service for something in the neighborhood of 250
to 500 domains.

Can anyone recommend a good company providing secondary DNS service?
I imagine I'd need some kind of a web-based management interface to
allow me to add and remove zones on the secondary.

Thanks,
Jim