The T/C Big Boar .58 is a great gun and highly sought after. Unfortunately it's impossible to ascertain or even guess at it's age; production records were lost in a fire and there is no database of what serial numbers were made when. As for value, it depends greatly, in fact almost entirely, on condition. Assuming good, used condition (95% is the term I'd use), the Blue Book of Modern Black Powder Arms (Fifth Edition 2007) lists a value of $160. A 100% condition example would bring nearly twice that, about $300.
The Blue Book is only a guide. Actual prices can and do vary considerably in either direction. I believe actual Big Boar prices tend to be higher than the Blue Book valuation by as much as $60-$75.