I think it will depend on how well it was done ,qulity of wood ,the gun itself and what was wrong with the original stock . Nothing looks worse than restocked good quality gun with an original worn forend wood .Unlike rebarreling by anotherit will only have a minoreffect and in some cases will actualy enhance the value.
For example I have recently re stocked a William Evans ,the original stock had been cut down to 12&1/4 inches ,it had then been extended twice . Was it worth more then or now with 15 inch stock in slightly better than original wood .[forend reovated and re checkered to match,of corse.] ?