Really beautiful bike you have there, probably from the mid 90s or thereabouts.
The saddle is an Avocet Spenco racing saddle which would date from about then. You can get new / old stock ones, or you can get Selle Italia Turbo modern reproductions which are that similar look, personally I think it looks "right" for that era and I wouldn't change the saddle type to a modern one.
I've no real idea of value. Depends if it's all genuine and original (and it seems it from those few pictures, the only thing throwing me is that Wilier tended and still tend towards brighter metallic colours, oranges, azure blues etc, and they tend to have Columbus tubing decals which I can't see - Resprayed at some point perhaps?) The groupset looks right, Campag Record (I think) from the 90s. If it's genuinely a Wilier it's worth a good amount.