I would say it worth approximately USD150 ~ 200+, 
depending on whether the symbol is very obvious 
on his head, if Tamahome's eyes is open and his 
costume. If it's the costume he wears when he 
first saw Miaka, it would be more. ^_^

But I guess if Tamahome bears Nakago symbol on 
his forehead, the pricing will certainly reach 
USD400 ~ 1000+ easily.