That depends on a lot.
Silver plated? Are you sure its not just nickeled?
Who engraved it?
Unless both of those questions can be answered with certainty, the value is likely not increased much, if at all, over a standard model.
Even if the gun was silver plated, it would have had to be done by a master engraver (doesn't look like it to me) to really add any value, and being an Astra, it wouldn't add much value..probably not even the cost of the engraving.
I would value that particular Astra at right around $400-$500 as it sits with no provenance. I like Astra's, but unfortunately they aren't collectible and money spent prettying one up is money wasted, imo.