Whether one die has any value will depend on the collector. Some collectors will buy singles, hoping that someday they will find a matching one...or thinking that having one is better than none. As a general rule, though, a single will sell for less than half of what a pair would sell for.

(Similar to some collectors who will only collect 'new' or 'very slightly used' chips, while others will start with used chips and upgrade later if possible. Some matchbook collectors will only collect full books, others will collect partial books or empty covers if that's all they can find....etc... )