I'm no expert, but at least your thread gets bumped to the top with my reply!
Generally speaking, engraving on firearms
typically only raises the "collector value" when the engraving was done from the manufacturing factory and as originally ordered in the original production of the gun. Typically, such engraving was offered many years ago and not so much recently. Due to the time, care, skill, etc involved, it has -NEVER- been a low-cost "option" and anything that cost more originally makes the firearm worth more now. Also tends to make them more rare since so few people could justify the (sometimes nearly irrational!) cost associated with it.
Bottom line is that typically, a firearm that's been engraved outside of the factory and wasn't typically ordered and shipped by the manufacturer as such is simply ornamental and has value only to a buyer who... likes it.
And, perhaps even more of a bummer, if the firearm was one that was highly collectible in the first place, aftermarket engraving well outside of the original manufacturer absolutely
hurts the collectible value.
It's very much like a rookie baseball card of a superstar player... with an autograph on it. Those who don't know collectible cards think
"WOW!" and those who do know collectible cards think "ugh...!" as the signature just destroys the collectible value of the card.
You don't have a 1930s-era Smith & Wesson Registered Magnum, so your engraving didn't attack the collectible value of a rare gun, but if it wasn't ordered as such from Smith & Wesson and shipped from S&W with that engraving (able to be proven as such with a S&W factory letter from Roy Jinks) then your engraving is, well, not helping the "collectible" value of the revolver.
To the right buyer who enjoys that sort of thing, it may garner serious interest. But generally speaking, it's a Model 66 with aftermarket engraving.
Now I know less than squat about engraving, but even an ultra-quick search on the name "Ben Shostle" suggests that the man was
extremely well known and respected in his field, and his work is sought after. So that's excellent news, for the value of the revolver and for piece of mind for what you spent.