I think there's some stuff in the "archives" of this forum. Basically, we don't really encourage it, but if you want to do it, there's a cost of $25 setup per nameserver (generally 2 or 3), plus a monthly fee for the IP. You don't add the IPs to your account as unique IPs, though - they come out of different subnets - the IPs and charges are added as part of the setup process. My guess is that some point we'll automate it further, but right now, the setup is a royal pain.
The other thing that's a pain is that new domains added to your domain won't default to your nameservers, so you'd have to write support every time you want to add a domain pointing to those nameservers.