There are two extinction coefficient values available:
Assuming all Cys residues form cystines - 1.935 at 280 nm in water, 0.1% (1 g/L)
Assuming all Cys residues are reduced - 1.929 at 280 nm in water, 0.1% (1 g/L)
Please see the link below to review this and additional information available at Expasy:
https://web.expasy.org/cgi-bin/protparam/protparam1?P00371@noft@