In my limited experience of engraving there is a difference between "value" and "purchase price".

The value is in the eyes of the purchaser.

Purchase price especially a new firearm will reflect the cost of the engraving, which can add substantially to the purchase price considerably.

Vintage rifles values reflects the rifle, brand, amount and quality of the engraving etc, and of course supply and demand ie the market.

Personally for me, the more engraving the more I can't afford the firearm. The reason I go for plainer rifles, even if I like quality engraving.