As mentioned in another reply above, I like 'domain name one'. Seems like any development along the lines of early domain name history, or talks about the most prime/important/valuable domain names, and you've got the prime name for a site like that. Dn1.com sounds more like the ordering code for an electronics part in your radio, and domainname1/one.com is long and clumsy; I like dnone very much, after getting past the initial misleading visual of seeing it as d-none.com
I agree with most above as to prices: xx for resellers, low xxx with minor development, high xxx to x,xxx with any even slightly interesting development.