What is the value of a Fender Telecaster guitar autographed by Bob Seger?

It is worth whatever the buyer is willing to pay. This worth is set by selling and buying.

If you offer a sale price of $1000 and the person is willing to pay that amount, then it is worth that much. Although if the buyer makes a counter offer of less and you accept, then that is the value. No actual dollar amount can be determined until a transaction has been made.