There is always value

In the USA, having the sideplate makes the value less. For me, for value, it would matter if all other parts are "matching". If this is the only part mismatching, then it would be worth more, than if the grips are mismatched, plus the sideplate, if the firing pin plus the sideplate is mismatched, etc.
No matter what, a mismatched sideplate makes a difference in value to most folks.
Ed