It’s impossible to give an exact number because French horns will vary a lot in value depending on several factors. You can never really depend on only two criteria such as the instrument's age and model. The final price depends on other things as well, including how well-maintained the instrument is for its age, the material used in manufacturing it, its brand, and if or not it's still playable. In order to determine an exact number, you'll have to take the instrument to an expert for a proper appraisal.