I can relate some experience based on antique pocket watches which may or may not apply. Good, engraving that was done by hand with varying line width and an artistic design has a neutral effect on the long term value. Mechanical engraving in the "Things Remembered" style would reduce the value to a parts watch (not collectible and minimal value). I know you said your gun is a common non-collectible model, but if you ever plan to sell it or trade it in, $20 engraving will negatively impact the value. You might consider getting a replacement side plate (if that's possible for your model) and have the engraving done on that. Then store the original side plate away in case you ever want to return it to stock.