IMO somewhere in the high $xx or low $xxx range would be realistic. ©.com presumably gets some type-ins by people playing around with Alt codes or what not, and the rarity of odd one-character domains (or their encoded equivalents) certainly adds a lot to its value.
In the distant future, though... who knows? Perhaps the character will start appearing on keyboards. For it to really become valuable I think the domain name system would have to expand its character set support so that users could see ©©.com instead of xn--***.com. So I'd say currently it has no end-user value, but could become valuable in later years.