The 925 hallmark denotes sterling silver. A gold-coloured pen bearing it is therefore, as a rule, gilded sterling silver, that is to say vermeil. Montblanc lists such versions under the name Solitaire: what you have is a Meisterstück Le Grand whose metal sleeve is executed in silver or vermeil. Two variants are common, namely pieces made entirely in precious metal and versions in which only the cap is silver or vermeil while the body is black precious resin. Where the hallmarks sit shows which parts are involved.
For establishing a value, the bare metal content is of little help. What counts are the prices actually realised for comparable pieces in comparable condition, that is, completed sales and not asking prices. The state of the gilding, scratches on the sleeve, completeness of box and papers and a mechanism in perfect working order all weigh far more heavily than the silver.