I would say that you have a very nice weapon, but nothing about it makes it particularly rare or collectable. 257 Roberts is a pretty popular caliber and the model and synthetic stock are very popular.
BUT, after going back and looking, I can't find that the ADL was offered in 257 Roberts. I find several BDL models that offered it. Are you sure of the ADL designation? It came in the Lew Horton Special Edition, which are not particularly expensive, and the 700BDL Classic which they put out the 257 in 1982. These are worth $500-$600 in excellent condition according to the BB.